So, I started to make a Flutter App with sdk: ">=2.16.2 <3.0.0" and Firebase Integration. I have the following code on main.dart for "authStateChanges()":

if (snapshot.connectionState == ConnectionState.done) {
   FirebaseAuth.instance.authStateChanges().listen((User? user) {
      if (user == null) {
         Navigator.of(context).pushReplacementNamed('login');
      } else {
         Navigator.of(context).pushReplacementNamed('home');
      }
   });
}

It works at the first time when loading the app, but If the user signs in (or out), then the "Unhandled Exception: Null check operator used on a null value" occurs on Navigator.

Here is the full main.dart code:

import 'package:firebase_auth/firebase_auth.dart';
import 'package:flutter/material.dart';
import 'package:firebase_core/firebase_core.dart';
import 'package:pollux_android/src/pages/error_page.dart';
import 'package:pollux_android/src/pages/loading_page.dart';
import 'package:pollux_android/src/routes/routes.dart';

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

class MyApp extends StatefulWidget {
  MyApp({Key? key}) : super(key: key);

  @override
  State<MyApp> createState() => _MyAppState();
  final GlobalKey<NavigatorState> navigatorKey = GlobalKey<NavigatorState>();
}

class _MyAppState extends State<MyApp> {
  final Future<FirebaseApp> _initialization = Firebase.initializeApp();
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      routes: polluxRoutes,
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: mainScreen(),
    );
  }

  mainScreen() {
    return FutureBuilder(
      future: _initialization,
      builder: (context, snapshot) {
        if (snapshot.hasError) {
          return const ErrorPage(
            errorMessage: 'Unknown error',
          );
        }

        if (snapshot.connectionState == ConnectionState.done) {
          FirebaseAuth.instance.authStateChanges().listen((User? user) {
            if (user == null) {
              Navigator.of(context).pushReplacementNamed('login');
            } else {
              Navigator.of(context).pushReplacementNamed('home');
            }
          });
        }

        return const LoadingPage();
      },
    );
  }
}

I don't get why it fails to Navigate the second time on authStateChanges(). I appreciate for the help you can provide. I believe it has to do with the Null check on newer versions as this issue didn't happen on sdk 2.7.0

CodePudding user response：

Split your file main.dart into two different files

Do not put any logic in this file main.dart

Split file is solution for your issue

main.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatefulWidget {
  MyApp({Key? key}) : super(key: key);

  @override
  State<MyApp> createState() => _MyAppState();
  final GlobalKey<NavigatorState> navigatorKey = GlobalKey<NavigatorState>();
}

class _MyAppState extends State<MyApp> {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      routes: {
        'home': (_) => HomeScreen(),
        
      },
      home: LoginScreen(),
    );
  }
}

login_screen.dart

import 'package:flutter/material.dart';

class LoginScreen extends StatefulWidget {
  const LoginScreen({Key? key}) : super(key: key);

  @override
  State<LoginScreen> createState() => _SplashScreenState();
}

class _SplashScreenState extends State<LoginScreen> {
  final Future<FirebaseApp> _initialization = Firebase.initializeApp();

  @override
  Widget build(BuildContext context) {
    return mainScreen();
  }

  mainScreen() {
    return FutureBuilder(
      future: _initialization,
      builder: (context, snapshot) {
        if (snapshot.hasError) {
          return const ErrorPage(
            errorMessage: 'Unknown error',
          );
        }

        if (snapshot.connectionState == ConnectionState.done) {
          FirebaseAuth.instance.authStateChanges().listen((User? user) {
            if (user == null) {
              Navigator.of(context).pushReplacementNamed('login');
            } else {
              Navigator.of(context).pushReplacementNamed('home');
            }
          });
        }

        return const LoadingPage();
      },
    );
  }
}

CodePudding user response：

For some reason the context becomes null when authStateChanges() happens. Thanks to @Saitoh-Akira, I got the idea of using navigator Key and use it to navigate without the context. If I understand this correctly, the Navigator needs a state. Anyway, adding the navigator Key and use it worked good:

import 'package:firebase_auth/firebase_auth.dart';
import 'package:flutter/material.dart';
import 'package:firebase_core/firebase_core.dart';
import 'package:pollux_android/src/pages/error_page.dart';
import 'package:pollux_android/src/pages/loading_page.dart';
import 'package:pollux_android/src/routes/routes.dart';

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

class MyApp extends StatefulWidget {
  MyApp({Key? key}) : super(key: key);

  @override
  State<MyApp> createState() => _MyAppState();
  // final GlobalKey<NavigatorState> navigatorKey = GlobalKey<NavigatorState>();
}

class _MyAppState extends State<MyApp> {
  final Future<FirebaseApp> _initialization = Firebase.initializeApp();
  final GlobalKey<NavigatorState> navigatorKey = GlobalKey<NavigatorState>();
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      routes: polluxRoutes,
      navigatorKey: navigatorKey,
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: mainScreen(),
    );
  }

  mainScreen() {
    return FutureBuilder(
      future: _initialization,
      builder: (context, snapshot) {
        if (snapshot.hasError) {
          return const ErrorPage(
            errorMessage: 'Some unknown error happened',
          );
        }

        if (snapshot.connectionState == ConnectionState.done) {
          FirebaseAuth.instance.authStateChanges().listen((User? user) {
            if (user == null) {
              navigatorKey.currentState?.pushNamedAndRemoveUntil('login', (route) => false);
            } else {
              navigatorKey.currentState?.pushNamedAndRemoveUntil('home', (route) => false);
            }
          });
        }

        return const LoadingPage();
      },
    );
  }
}