TypeError: Cannot read properties of null (reading 'email')

Question

有人知道我为什么会收到这个错误吗？我正在使用 final TextEditingController _emailController = TextEditingController(); final TextEditingController _passwordController = TextEditingController(); 从文本字段（密码和电子邮件）中获取文本。

然后我将 signIn() 方法与电子邮件一起使用：_emailController.text.trim(), password: _passwordController.text.trim() 从文本字段中获取字符串。此信息用于登录用户。

按钮登录只使用methodde signIn() 然后显示页面仪表板。在页面仪表板上有一个包含登录用户电子邮件的文本。

为什么会出现这个错误？我哪里做错了？

错误信息：

======== Exception caught by widgets library =======================================================
The following JSNoSuchMethodError was thrown building DashboardView(dirty, state: _DashboardViewState#00d1f):
TypeError: Cannot read properties of null (reading 'email')

The relevant error-causing widget was: 
  DashboardView DashboardView:file:///C:/Users/Dennis/Documents/bestfitnesstrackereu/lib/routing/router.dart:25:28
When the exception was thrown, this was the stack: 
packages/bestfitnesstrackereu/pages/dashboard/dashboard_view.dart 50:1                                            build
packages/flutter/src/widgets/framework.dart 4922:27                                                               build

authentication.dart代码：（登录）

import 'package:bestfitnesstrackereu/routing/route_names.dart';
import 'package:firebase_auth/firebase_auth.dart';
import 'package:flutter/material.dart';

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

  @override
  State<AuthenticationPage> createState() => _AuthenticationPageState();
}

class _AuthenticationPageState extends State<AuthenticationPage> {
  bool checkBoxValue = false;
  //textfield controllers:
  final TextEditingController _emailController = TextEditingController();
  final TextEditingController _passwordController = TextEditingController();


  Future signIn() async{
    try {
      await FirebaseAuth.instance.signInWithEmailAndPassword(
          email: _emailController.text.trim(),
          password: _passwordController.text.trim()
      );
    } on FirebaseAuthException catch (e){
      print(e);
    }
  }

  @override
  void dispose() {
    _emailController.dispose();
    _passwordController.dispose();
    super.dispose();
  }


  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Container(
          constraints: BoxConstraints(maxWidth: 400),
          padding: EdgeInsets.all(24),
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: [
TextField(
                controller: _emailController,
                decoration: InputDecoration(
                  labelText: "E-Mail",
                  hintText: "abc@domain.com",
                    suffixIcon: Icon(Icons.mail_outline,),
                  border: OutlineInputBorder(
                    borderRadius: BorderRadius.circular(20)
                  )
                ),
              ),

              SizedBox(height: 15,),

TextField(
                controller: _passwordController,
                obscureText: true,
                decoration: InputDecoration(
                    labelText: "Password",
                    hintText: "******",
                    suffixIcon: Icon(Icons.lock_outline, color: Colors.grey,),
                    border: OutlineInputBorder(
                        borderRadius: BorderRadius.circular(20)
                    )
                ),
              ),
InkWell(
                onTap: (){
                  signIn();
                  Navigator.of(context).pushNamed(DashboardRoute);
                },
                child: Container(
                  decoration: BoxDecoration(color: Colors.deepPurple,
                  borderRadius: BorderRadius.circular(20)),
                  alignment: Alignment.center,
                  width: double.maxFinite,
                  padding: EdgeInsets.symmetric(vertical: 16),
                  child: Text(
                    "Login",
                    style: TextStyle(
                    color: Colors.white,
                  ),)
              )
              ),
 ],
          ),
        )
      ),
    );
  }
}

dashboard_view.dart代码：

import 'package:firebase_auth/firebase_auth.dart';
import 'package:flutter/material.dart';  
import '../../routing/route_names.dart';

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

  @override
  State<DashboardView> createState() => _DashboardViewState();
}

class _DashboardViewState extends State<DashboardView> {
  final user = FirebaseAuth.instance.currentUser;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Column(
          mainAxisAlignment:  MainAxisAlignment.center,
          children: [
            Text('Dashboard View - signed in as '),
            SizedBox(height: 8,),
            Text(user.email),
            InkWell(
                onTap: () async {
                  await FirebaseAuth.instance.signOut();
                  print('user ist ausgeloggt');
                  //Navigator.of(context).pushNamed(AuthenticationPageRoute);
                },
                child: Container(
                    decoration: BoxDecoration(color: Colors.deepPurple,
                        borderRadius: BorderRadius.circular(20)),
                    alignment: Alignment.center,
                    width: double.maxFinite,
                    padding: EdgeInsets.symmetric(vertical: 16),
                    child: Text(
                      'sign out',
                      style: TextStyle(
                        color: Colors.white,
                      ),)
                )
            ),


          ],
        ),
    ),
    );
  }
}

Answer 1

你在做

onTap: (){
              signIn();
              Navigator.of(context).pushNamed(DashboardRoute);
            },

由于您没有等待 signIn() 方法完成，您已经导航到 DashboardRoute 并且 _emailController 已处理。

这就是您遇到问题的原因。

在导航到下一个屏幕之前尝试等待标志的响应

onTap: () async {
              await signIn();
              Navigator.of(context).pushNamed(DashboardRoute);
            },