I took a course about Flutter from udemy. I've come to the Firebase part. However, I am getting errors in this part. I am getting an error even though I type exactly the same. I think I got error like this because there are updates in Flutter language. I'm sorry my English is bad. Thank you in advance for helping me.

enter image description here

main.dart

import 'dart:html';

import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:fire_base1/user.dart';
import 'package:firebase_core/firebase_core.dart';
import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: const MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({Key? key, required this.title}) : super(key: key);

  final String title;

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  Future<List<User>> getData() async {
    final res = await FirebaseFirestore.instance.collection("users").get();

    List<User> myresult = <User>[];
    for (int i = 0; i < res.docs.length; i  ) {
      User user = new User(
          name: res.docs[i]["name"],
          last_name: res.docs[i]["last_name"],
          email: res.docs[i]["email"]);
      myresult.add(user);
    }
    return myresult;
  }

  int _counter = 0;

  void _incrementCounter() {
    setState(() {
      _counter  ;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text("FireBase"),
        ),
        body: FutureBuilder(
          future: Firebase.initializeApp(),
          builder: (context, snapshot) {
            if (snapshot.hasError) {
              return Text("Veritabanına bağlanırken bir hata meydana geldi");
            }
            if (snapshot.connectionState == ConnectionState.done) {
              return FutureBuilder(
                future: getData(),
                builder: (context, snapshot) {
                  if (snapshot.connectionState == ConnectionState.waiting) {
                    return Center(
                      child: CircularProgressIndicator(),
                    );
                  } else if (snapshot.connectionState == ConnectionState.done) {
                    return ListView.builder(
                      itemCount: snapshot.data.length,
                      itemBuilder: (context, index) {
                        return ListTile(
                          title: Text(snapshot.data[index].name  
                              " "  
                              snapshot.data[index].last_name),
                          subtitle: Text(snapshot.data[index].email),
                        );
                      },
                    );
                  } else {
                    return SizedBox();
                  }
                },
              );
            }
            return Center(
              child: CircularProgressIndicator(),
            );
          },
        ));
  }
}

user.dart

class User {
  String name;
  String last_name;
  String email;

  User({required this.name, required this.last_name, required this.email});

  Map<String, dynamic> toMap() {
    Map<String, dynamic> data = new Map<String, dynamic>();
    data["name"] = name;
    data["last_name"] = last_name;
    data["email"] = email;
    return data;
  }
}

CodePudding user response：

Please always copy/paste the error your IDE or your runtime shows in here. Screenshots won't help much.

Anyways, by looking at your image it seems like that data is a nullable variable, and therefore accessing it throws a compile-time error because of sound null safety.

You have two options:

1. Riskier and inelegant hasty option: just add a bang operator ! to perform a null check, in which you're telling Dart that you're sure that data isn't null
2. Robust, elegant, well-thought option: learn and use the null-aware operators such as ?., ?? and ??=.

For example in the following snippet (by looking at your code):

itemCount: snapshot.data.length

I'd do something like:

itemCount: snapshot.data?.length ?? 0

Which solution might not be correct - it depends on your business logic, i.e. what you want to do.

The inelegant option would be something like

if (data != null)
    itemCount: snapshot.data!.length

Which can lead to unexpected runtime errors in the long run.