例外：NoSuchMethodError：在 null 上调用了 getter 'uid'。接收方：null 尝试调用：uid答案

【问题标题】：Exception: NoSuchMethodError: The getter 'uid' was called on null. Receiver: null Tried calling: uid例外：NoSuchMethodError：在 null 上调用了 getter 'uid'。接收方：null 尝试调用：uid
【发布时间】：2021-07-27 10:02:07
【问题描述】：

我在 Flutter 中使用 Firebase 作为后端，并且在用户身份验证中引发异常。每当我启动应用程序时，异常就会发生一次。

当我尝试注销时，异常发生一次，但屏幕没有改变。如果我重新启动应用程序，屏幕会更改并且用户已注销，但现在会显示两次异常。如果我在此错误处继续重新启动应用程序，则应用程序的其余部分工作正常。

每当我尝试退出时，这就是显示的输出

D/FirebaseAuth(23343): Notifying id token listeners about a sign-out event.
D/FirebaseAuth(23343): Notifying auth state listeners about a sign-out event.
W/y_personal_che(23343): Accessing hidden method Lsun/misc/Unsafe;->getObject(Ljava/lang/Object;J)Ljava/lang/Object; (greylist, linking, allowed)

======== Exception caught by provider ==============================================================
The following assertion was thrown:
An exception was throw by _MapStream<User?, Userc> listened by

StreamProvider<Userc>, but no `catchError` was provided.

Exception:
NoSuchMethodError: The getter 'uid' was called on null.
Receiver: null
Tried calling: uid

====================================================================================================

这是我的验证码：

import 'package:firebase_auth/firebase_auth.dart';
import 'package:flutter/material.dart';
import 'package:my_personal_chef/Models/user.dart';
import 'package:my_personal_chef/services/database.dart';

class AuthService{

  final FirebaseAuth _auth = FirebaseAuth.instance;
  String uid;


  //creating a user object from firebase user
  Userc _userfromFirebaseUser (User user) {
    print(user.uid);
   return user!=null ? Userc(uid: user.uid, Email: user.email,) : null;

  }

  //auth change user stream
  Stream <Userc> get user{
    return _auth.authStateChanges()
        .map((User user) => _userfromFirebaseUser(user));
  }
  //anonymous sign in
  Future signinanon() async{
    try{UserCredential result =  await _auth.signInAnonymously();
    final User user = result.user;

    uid = user.uid;
    print(uid);
    await DatabaseService(userId: uid).addUser(_userfromFirebaseUser(user));

    return _userfromFirebaseUser(user);
    } catch(e){
      print(e.toString());
      return null;
    }

  }
  //sign in with email and password
  Future signinwithemailandpassword (String email, String password) async{
    try{
      UserCredential result = await _auth.signInWithEmailAndPassword(email: email, password: password);
      final User user = result.user;

      return _userfromFirebaseUser(user);
    }
    catch(e){
      print(e.toString());
    }
  }
  //register with email and password
  Future registerwithemailandpassword(String email, String password)async{
    try{
      UserCredential result = await _auth.createUserWithEmailAndPassword(email: email, password: password);
      final User user = result.user;

      uid = user.uid;
      print(uid);
      await DatabaseService(userId: uid).addUser(_userfromFirebaseUser(user));
      return _userfromFirebaseUser(user);

    }
    catch(e){
      print(e.toString());
      return null;
    }
  }

  String getUserbyId() {
    return  _auth.currentUser.uid;

  }

  //Sign out
  Future signout() async{
    try{
      return await _auth.signOut();
    }catch(e){
      print(e.toString());
      return null;
    }
  }
}

这是我的 SignOut 按钮 onPressed 属性：

 onPressed: ()async{
                  await _auth.signout();
                },

其中，最终的 AuthService _auth = AuthService();

这是我的项目文件。此外，欢迎任何贡献。 https://github.com/rajoriaakash/My-Personal-Chef

【问题讨论】：

这里是 ` return user!=null ？ Userc(uid: user.uid, Email: user.email,) : null;` from _userfromFirebaseUser 你正在映射到 null
这只是一个将 Firebase 用户数据转换为我的自定义用户模型数据的函数。我如何在这里映射到 null？

标签： firebase flutter android-studio firebase-authentication

【解决方案1】：

Stream <Userc> get user{
    return _auth.authStateChanges()
        .map((User user) => _userfromFirebaseUser(user));
  }

我认为问题出在这里，在某些版本的 firebase sdk 中，用户最初来自 authStateChanges。您最好检查一下 null 并找到最初不为 null 的正确版本。

firebase_auth: ^1.1.1这个版本适合我。

【讨论】：