NavHostFragment initialization results in NullPointerException-CodePudding

I am attempting to set a navHostFragment in my MainActivity.kt file, but the code below results in the error:

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.lunchtray/com.example.lunchtray.MainActivity}: java.lang.NullPointerException: null cannot be cast to non-null type androidx.navigation.fragment.NavHostFragment

class MainActivity : AppCompatActivity() {

    private lateinit var navController: NavController

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        val navHostFragment = supportFragmentManager // NULL CANNOT BE CAST TO NON NULL TYPE
            .findFragmentById(R.id.nav_host_fragment) as NavHostFragment
        navController = navHostFragment.navController

        setupActionBarWithNavController(navController)
    }

    override fun onSupportNavigateUp(): Boolean {
        return navController.navigateUp() || super.onSupportNavigateUp()
    }
}

activity_main.xml layout file:

<androidx.constraintlayout.widget.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:id="@ id/container"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <androidx.fragment.app.FragmentContainerView
        android:id="@ id/nav_host_fragment"
        android:name="androidx.navigation.fragment.NavHostFragment"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:defaultNavHost="true"
        app:navGraph="@navigation/mobile_navigation"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>

What am I doing wrong? Similar questions exist on SO but they involve using menu IDs or initializing the navHostFragment from within a fragment, this is a much more straightforward situation and I do not understand why any value should be null.

CodePudding user response：

You forgot to call setContentView(R.layout.activity_main), which means you have no FragmentContainerView and no fragment within it, hence why findFragmentById(R.id.nav_host_fragment) returns null.

    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_main)

    val navHostFragment = supportFragmentManager
        .findFragmentById(R.id.nav_host_fragment) as NavHostFragment
    navController = navHostFragment.navController

As an alternative, you can also call the AppCompatActivity construct that takes a LayoutRes, which will automatically call setContentView for you as part of super.onCreate():

class MainActivity : AppCompatActivity(R.ayout.activity_main) {