Kotlin based Android app crash when changing activity using a button-CodePudding

I am building my first app and i am trying to change activities using buttons.The first button works fine. But the second crashes the app giving this error: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference I tried researching, but i didn't find a working solution. So please help me out here

login.kt

package com.example.login

import android.content.Intent
import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.widget.Button

class login : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_login)

        val  loginbutton = findViewById<Button>(R.id.signinbtn)

        loginbutton.setOnClickListener {
            val intent = Intent(this,gamefeed::class.java)
            startActivity(intent)
            finish()

            }

        val  signupbutton = findViewById<Button>(R.id.signupbtn)

        signupbutton.setOnClickListener {
            val intent = Intent(this,Registration::class.java)
            startActivity(intent)
            finish()
        }

    }

}

activity_login.xml

 <?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@ id/activity_login"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity"
    android:background="@drawable/ic_spalsh"
    tools:ignore="InvalidId">



    <androidx.constraintlayout.widget.ConstraintLayout
        android:id="@ id/constraintLayout"
        android:layout_width="392dp"
        android:layout_height="217dp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent">

        <ImageView
            android:id="@ id/imageView2"
            android:layout_width="399dp"
            android:layout_height="228dp"
            android:layout_marginBottom="32dp"
            android:contentDescription="@string/todo"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="parent"
            app:layout_constraintVertical_bias="0.285"
            app:srcCompat="@drawable/login1" />

    </androidx.constraintlayout.widget.ConstraintLayout>

    <TextView
        android:id="@ id/textView3"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="4dp"
        android:text="@string/Join_with_us"
        android:textColor="@color/black"
        android:textSize="34sp"
        android:textStyle="bold"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.498"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@ id/constraintLayout"
        tools:ignore="TextContrastCheck" />

    <LinearLayout
        android:id="@ id/linearLayout"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginStart="30dp"
        android:layout_marginTop="30dp"
        android:layout_marginEnd="30dp"
        android:background="@drawable/background_edittext"
        android:elevation="8dp"
        android:orientation="vertical"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@ id/textView3">



        <com.google.android.material.textfield.TextInputEditText
            android:id="@ id/editTextTextPersonName"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:background="@null"
            android:ems="10"
            android:hint="@string/username"
            android:importantForAutofill="no"
            android:inputType="textPersonName|textEmailAddress"
            android:padding="24dp"
            tools:ignore="TextContrastCheck"
            />

        <View
            android:layout_width="match_parent"
            android:layout_height="1dp"
            android:layout_marginLeft="5dp"
            android:layout_marginRight="5dp"
            android:background="#EEEEEF"
            />

        <com.google.android.material.textfield.TextInputEditText

            android:id="@ id/editTextTextPassword"
            android:layout_width="match_parent"
            android:layout_height="71dp"
            android:ems="10"
            android:hint="@string/password"
            android:importantForAutofill="no"
            android:inputType="textPassword"
            android:padding="24dp" />
    </LinearLayout>

    <TextView
        android:id="@ id/textView4"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="20dp"
        android:text="@string/forgot_password"
        android:textColor="@color/purple_700"
        android:textSize="16sp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@ id/linearLayout"
        tools:ignore="TextContrastCheck" />

    <Button
        android:id="@ id/signupbtn"
        style="@style/Widget.AppCompat.Button.Borderless"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="100dp"
        android:backgroundTint="@color/white"
        android:text="@string/Dont_have_account"
        android:textColor="@color/black"
        android:textSize="14sp"
        android:visibility="visible"
        app:cornerRadius="50dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@ id/signinbtn"
        tools:visibility="visible" />

    <Button
        android:id="@ id/signinbtn"
        android:layout_width="176dp"
        android:layout_height="68dp"
        android:layout_marginTop="24dp"
        android:background="@drawable/background_btn"
        android:backgroundTint="@color/bottom_nav_background"
        android:padding="15dp"
        android:text="@string/login"
        android:textColor="@color/white"
        android:textSize="20sp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.496"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@ id/textView4"
        app:layout_constraintWidth_percent="0.6"
        tools:ignore="DuplicateSpeakableTextCheck" />

</androidx.constraintlayout.widget.ConstraintLayout>

Registration.kt

import android.content.Intent
import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.widget.Button

class Registration : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_registration)

        val  signinbutton2 = findViewById<Button>(R.id.loginbtn)

        signinbutton2.setOnClickListener {
            val intent = Intent(this,login::class.java)
            startActivity(intent)
            finish()
        }
    }
}

activity_registration.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@ id/activity_registration"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".Registration"
    android:background="@drawable/ic_spalsh">

    <androidx.constraintlayout.widget.ConstraintLayout
        android:id="@ id/constraintLayout3"
        android:layout_width="393dp"
        android:layout_height="213dp"
        android:background="@drawable/registration1"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHeight_percent="0.40"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent">

    </androidx.constraintlayout.widget.ConstraintLayout>

    <TextView
        android:id="@ id/textView6"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="16dp"
        android:text="@string/Register_Here"
        android:textColor="@color/black"
        android:textSize="28sp"
        android:textStyle="bold"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@ id/constraintLayout3"
        tools:ignore="TextContrastCheck" />

    <LinearLayout
        android:id="@ id/linearLayout2"
        android:layout_width="356dp"
        android:layout_height="354dp"
        android:layout_marginStart="30dp"
        android:layout_marginTop="12dp"
        android:layout_marginEnd="30dp"
        android:background="@drawable/background_edittext"
        android:orientation="vertical"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.263"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@ id/textView6">

        <EditText
            android:id="@ id/email"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:background="@null"
            android:ems="10"
            android:hint="@string/email"
            android:importantForAutofill="no"
            android:inputType="textPersonName|textEmailAddress"
            android:padding="10dp"
            tools:ignore="TouchTargetSizeCheck,TextContrastCheck" />

        <View
            android:layout_width="match_parent"
            android:layout_height="1dp"
            android:layout_marginLeft="5dp"
            android:layout_marginRight="5dp"
            android:background="#EEEEEF" />

        <EditText
            android:id="@ id/username"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:background="@null"
            android:ems="10"
            android:hint="@string/username"
            android:inputType="textPersonName"
            android:padding="10dp"
            tools:ignore="TouchTargetSizeCheck,TextContrastCheck"
            android:importantForAutofill="no" />

        <View
            android:layout_width="match_parent"
            android:layout_height="1dp"
            android:layout_marginLeft="5dp"
            android:layout_marginRight="5dp"
            android:background="#EEEEEF" />

        <EditText
            android:id="@ id/age"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:background="@null"
            android:ems="10"
            android:hint="@string/age"
            android:inputType="textPersonName"
            android:padding="10dp"
            tools:ignore="TextContrastCheck,TouchTargetSizeCheck"
            android:importantForAutofill="no" />

        <View
            android:layout_width="match_parent"
            android:layout_height="1dp"
            android:layout_marginLeft="5dp"
            android:layout_marginRight="5dp"
            android:background="#EEEEEF" />

        <TextView
            android:id="@ id/gender"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_marginTop="5dp"
            android:background="@null"
            android:text="@string/gender"
            android:textColor="#AEA8BC"
            android:textColorHighlight="@color/purple_700"
            android:textSize="20sp"
            tools:ignore="TextContrastCheck" />

        <RadioGroup
            android:layout_width="match_parent"
            android:layout_height="44dp"
            android:orientation="horizontal">

            <RadioButton
                android:id="@ id/malerb"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:hint="@string/male"
                tools:ignore="TouchTargetSizeCheck" />

            <RadioButton
                android:id="@ id/femalerb"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_marginStart="40dp"
                android:hint="@string/female"
                tools:ignore="TextContrastCheck,TouchTargetSizeCheck" />

            <RadioButton
                android:id="@ id/otherrb"
                android:layout_width="128dp"
                android:layout_height="wrap_content"
                android:layout_marginStart="40dp"
                android:layout_weight="1"
                android:hint="@string/other"
                tools:ignore="TouchTargetSizeCheck" />
        </RadioGroup>

        <View
            android:layout_width="match_parent"
            android:layout_height="1dp"
            android:layout_marginLeft="5dp"
            android:layout_marginRight="5dp"
            android:background="#EEEEEF" />

        <EditText
            android:id="@ id/password"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:ems="10"
            android:hint="@string/password1"
            android:inputType="textPassword"
            android:padding="10dp"
            tools:ignore="TouchTargetSizeCheck"
            android:importantForAutofill="no" />

        <View
            android:layout_width="match_parent"
            android:layout_height="1dp"
            android:layout_marginLeft="5dp"
            android:layout_marginRight="5dp"
            android:background="#EEEEEF" />

        <EditText
            android:id="@ id/confirmpassword"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:ems="10"
            android:hint="@string/confirm_password"
            android:inputType="textPassword"
            android:padding="10dp"
            tools:ignore="TextContrastCheck,TouchTargetSizeCheck"
            android:importantForAutofill="no" />

        <View
            android:layout_width="match_parent"
            android:layout_height="1dp"
            android:layout_marginLeft="5dp"
            android:layout_marginRight="5dp"
            android:background="#EEEEEF" />

        <Button
            android:id="@ id/regbtn"
            android:layout_width="131dp"
            android:layout_height="wrap_content"
            android:layout_marginStart="105dp"
            android:background="@drawable/background_btn"
            android:textColor="@color/white"
            android:text="@string/register"
            tools:ignore="DuplicateSpeakableTextCheck" />

    </LinearLayout>

     <Button
        android:id="@ id/signin"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="8dp"
        android:layout_marginBottom="123dp"
        android:background="#00FFFFFF"
        android:text="@string/Allready_have_account"
        android:textColor="@color/black"
        android:textSize="16sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.497"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@ id/linearLayout2"
        app:layout_constraintVertical_bias="0.0" />

</androidx.constraintlayout.widget.ConstraintLayout>

Error

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.login/com.example.login.Registration}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3449)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:223)
        at android.app.ActivityThread.main(ActivityThread.java:7656)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
        at com.example.login.Registration.onCreate(Registration.kt:15)
        at android.app.Activity.performCreate(Activity.java:8000)
        at android.app.Activity.performCreate(Activity.java:7984)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1309)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3422)

CodePudding user response：

You get an error because there is no loginbtn on the activity_registration.xml page

class Registration : AppCompatActivity() {
    private lateinit var regbtn:Button
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_registration)

        regbtn=findViewById<Button>(R.id.regbtn)

        signinbutton2.setOnClickListener {
            val intent = Intent(this,login::class.java)
            startActivity(intent)
            finish()
        }
    }

I recommend you to look at the binding structure

build.gradle(module)

buildFeatures {
        viewBinding true
        dataBinding true
    }

Registration.kt

    class Registration : AppCompatActivity() {
    private lateinit var binding: RegistrationBinding

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding = ActivityAdminBinding.inflate(layoutInflater)
        val view=binding.root
        setContentView(view)
        binding.signinbutton2.setOnClickListener {
            val intent = Intent(this,login::class.java)
            startActivity(intent)
            finish()
        }
    }
}