Flutter Firebase邮箱密码登录报错：Null check operator used on a null value

错误信息 #

Exception has occurred.
_CastError (Null check operator used on a null value)

问题描述 #

调用Firebase认证服务的signInWithEmailAndPassword方法时抛出上述异常，无法定位具体错误位置，已确认问题出在认证环节。

存在问题的函数 #

Future<User?> signInWithEmailPassword(String email, String password) async {
  await Firebase.initializeApp();
  User? user;

  try {
    UserCredential userCredential = await _auth.signInWithEmailAndPassword(
      email: email,
      password: password,
    );
    user = userCredential.user;

    if (user != null) {
      uid = user.uid;
      userEmail = user.email;

      SharedPreferences prefs = await SharedPreferences.getInstance();
      await prefs.setBool('auth', true);
     // prefs.setString('id', user.uid);
    }
  } on FirebaseAuthException catch (e) {
    if (e.code == 'user-not-found') {
      print('No user found for that email.');
    } else if (e.code == 'wrong-password') {
      print('Wrong password provided.');
    }
  }

  return user;
}

完整认证模块代码 #

import 'package:firebase_auth/firebase_auth.dart';
import 'package:firebase_core/firebase_core.dart';
import 'package:google_sign_in/google_sign_in.dart';
import 'package:shared_preferences/shared_preferences.dart';
import 'package:flutter/foundation.dart' show kIsWeb;

final FirebaseAuth _auth = FirebaseAuth.instance;
final GoogleSignIn googleSignIn = GoogleSignIn();

String? uid;
String? name;
String? userEmail;
String? imageUrl;

/// For checking if the user is already signed into the
/// app using Google Sign In
Future getUser() async {
  await Firebase.initializeApp();

  SharedPreferences prefs = await SharedPreferences.getInstance();
  bool? authSignedIn = prefs.getBool('auth');
  print(authSignedIn);

  final User? user = _auth.currentUser;

  if (authSignedIn == true) {
    if (user != null) {
      uid = user.uid;
      name = user.displayName;
      userEmail = user.email;
      imageUrl = user.photoURL;
      prefs.setString('name', name!);
      prefs.setString('id', uid!);
    }
  }
}

/// For authenticating user using Google Sign In
/// with Firebase Authentication API.
///
/// Retrieves some general user related information
/// from their Google account for ease of the login process
Future<User?> signInWithGoogle() async {
  await Firebase.initializeApp();

  User? user;

  if (kIsWeb) {
    GoogleAuthProvider authProvider = GoogleAuthProvider();

    try {
      final UserCredential userCredential =
          await _auth.signInWithPopup(authProvider);

      user = userCredential.user;
    } catch (e) {
      print(e);
    }
  } else {
    final GoogleSignIn googleSignIn = GoogleSignIn();

    final GoogleSignInAccount? googleSignInAccount =
        await googleSignIn.signIn().catchError((onError) => print(onError));;

    if (googleSignInAccount != null) {
      final GoogleSignInAuthentication googleSignInAuthentication =
          await googleSignInAccount.authentication;

      final AuthCredential credential = GoogleAuthProvider.credential(
        accessToken: googleSignInAuthentication.accessToken,
        idToken: googleSignInAuthentication.idToken,
      );

      try {
        final UserCredential userCredential =
            await _auth.signInWithCredential(credential);

        user = userCredential.user;
      } on FirebaseAuthException catch (e) {
        if (e.code == 'account-exists-with-different-credential') {
          print('The account already exists with a different credential.');
        } else if (e.code == 'invalid-credential') {
          print('Error occurred while accessing credentials. Try again.');
        }
      } catch (e) {
        print(e);
      }
    }
  }

  if (user != null) {
    uid = user.uid;
    name = user.displayName;
    userEmail = user.email;
    imageUrl = user.photoURL;

    SharedPreferences prefs = await SharedPreferences.getInstance();
    prefs.setBool('auth', true);
    //prefs.setString('id', user.uid);
    //prefs.setString('name', user.displayName!);
  }

  return user;
}

Future resetPassword(String email) async {
  try {
    print(email);
    await FirebaseAuth.instance.sendPasswordResetEmail(email: email);
  } on FirebaseAuthException catch(e) {
    print(e);
    return false;
  }
  return true;
}

Future<User?> registerWithEmailPassword(String email, String password) async {
  await Firebase.initializeApp();
  User? user;

  try {
    UserCredential userCredential = await _auth.createUserWithEmailAndPassword(
      email: email,
      password: password,
    );

    user = userCredential.user;

    if (user != null) {
      uid = user.uid;
      userEmail = user.email;

      //SharedPreferences prefs = await SharedPreferences.getInstance();
      //prefs.setString('id', user.uid);
      //prefs.setString('name', user.displayName!);
    }
  } on FirebaseAuthException catch (e) {
    if (e.code == 'weak-password') {
      print('The password provided is too weak.');
    } else if (e.code == 'email-already-in-use') {
      print('The account already exists for that email.');
    }
  } catch (e) {
    print(e);
  }

  return user;
}

Future<User?> signInWithEmailPassword(String email, String password) async {
  await Firebase.initializeApp();
  User? user;

  try {
    UserCredential userCredential = await _auth.signInWithEmailAndPassword(
      email: email,
      password: password,
    );
    user = userCredential.user;

    if (user != null) {
      uid = user.uid;
      userEmail = user.email;

      SharedPreferences prefs = await SharedPreferences.getInstance();
      await prefs.setBool('auth', true);
     // prefs.setString('id', user.uid);
    }
  } on FirebaseAuthException catch (e) {
    if (e.code == 'user-not-found') {
      print('No user found for that email.');
    } else if (e.code == 'wrong-password') {
      print('Wrong password provided.');
    }
  }

  return user;
}

Future<String> signOut() async {
  await _auth.signOut();

  SharedPreferences prefs = await SharedPreferences.getInstance();
  prefs.setBool('auth', false);

  uid = null;
  userEmail = null;

  return 'User signed out';
}

/// For signing out of their Google account
void signOutGoogle() async {
  await googleSignIn.signOut();
  await _auth.signOut();

  SharedPreferences prefs = await SharedPreferences.getInstance();
  prefs.setBool('auth', false);

  uid = null;
  name = null;
  userEmail = null;
  imageUrl = null;

  print("User signed out of Google account");
}

解决方案 #

1. 修正Firebase初始化逻辑 #

Firebase初始化只需在App启动时执行一次，重复调用会导致实例异常。修改main函数：

void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await Firebase.initializeApp();
  runApp(MyApp());
}

删除所有认证函数中的await Firebase.initializeApp();语句。

2. 捕获所有异常并定位错误行 #

当前代码仅捕获FirebaseAuthException，Null check错误属于未捕获的Error，扩展catch范围并打印堆栈：

try {
  UserCredential userCredential = await _auth.signInWithEmailAndPassword(
    email: email,
    password: password,
  );
  // ... 后续代码
} on FirebaseAuthException catch (e) {
  // 原有错误处理
} catch (e) {
  print('异常详情: $e');
  print(e.stackTrace); // 打印堆栈，精准定位错误行
}

3. 修复潜在的Null检查风险 #

getUser函数中name可能为null（邮箱注册用户无displayName），直接使用name!会触发异常，添加非空判断：

if (user != null) {
  uid = user.uid;
  name = user.displayName;
  userEmail = user.email;
  imageUrl = user.photoURL;
  if (name != null) {
    prefs.setString('name', name!);
  }
  prefs.setString('id', uid!);
}

4. 验证_auth实例有效性 #

通过步骤1的初始化调整，确保全局_auth实例在Firebase初始化完成后才被调用，避免实例为null的情况。

内容的提问来源于stack exchange，提问作者Promaster