错误：预期类型的​​值＆＃x27; String＆＃x27;，但获得了类型之一。 （在颤音中）

Question

我正在尝试使用nodejs和mongodb在flutter应用程序（具有登录页面和注册页面）中对用户的登录信息进行身份验证，但是每当我输入有效的用户并按下登录按钮时，我会收到以下错误：

    Launching lib\main.dart on Chrome in debug mode...
This app is linked to the debug service: ws://127.0.0.1:50690/e09XNGpEkmc=/ws
Debug service listening on ws://127.0.0.1:50690/e09XNGpEkmc=/ws
 Running with sound null safety
Connecting to VM Service at ws://127.0.0.1:50690/e09XNGpEkmc=/ws
Flutter Web Bootstrap: Auto
All validations passed
Error: Expected a value of type 'String', but got one of type 'Null'
    at Object.throw_ [as throw] (http://localhost:50635/dart_sdk.js:5080:11)
    at Object.castError (http://localhost:50635/dart_sdk.js:5039:15)
    at Object.cast [as as] (http://localhost:50635/dart_sdk.js:5356:17)
    at String.as (http://localhost:50635/dart_sdk.js:46240:19)
at authservice.AuthService.new.login (http://localhost:50635/packages/flutterform/authservice.dart.lib.js:39:67)
    at login.throw (<anonymous>)
    at http://localhost:50635/dart_sdk.js:40646:38
    at _RootZone.runBinary (http://localhost:50635/dart_sdk.js:40515:59)
    at _FutureListener.thenAwait.handleError (http://localhost:50635/dart_sdk.js:35449:33)
    at handleError (http://localhost:50635/dart_sdk.js:36015:51)
    at _Future._propagateToListeners (http://localhost:50635/dart_sdk.js:36041:17)
    at [_completeError] (http://localhost:50635/dart_sdk.js:35878:23)
    at [_completeError] (http://localhost:50635/dart_sdk.js:35358:36)
    at _SyncCompleter.new.completeError (http://localhost:50635/dart_sdk.js:35282:29)
    at onError (http://localhost:50635/dart_sdk.js:35122:49)
    at _RootZone.runBinary (http://localhost:50635/dart_sdk.js:40515:59)
    at _FutureListener.then.handleError (http://localhost:50635/dart_sdk.js:35449:33)
    at handleError (http://localhost:50635/dart_sdk.js:36015:51)
    at _Future._propagateToListeners (http://localhost:50635/dart_sdk.js:36041:17)
    at [_completeError] (http://localhost:50635/dart_sdk.js:35878:23)
    at async._AsyncCallbackEntry.new.callback (http://localhost:50635/dart_sdk.js:35927:31)
    at Object._microtaskLoop (http://localhost:50635/dart_sdk.js:40778:13)
    at _startMicrotaskLoop (http://localhost:50635/dart_sdk.js:40784:13)
    at http://localhost:50635/dart_sdk.js:36261:9
Application finished.
Exited (sigterm)

我正在运行代码。在VSCODE编辑器中，我的服务器正在Heroku运行。 以下是我的代码：

从main.dart：

import 'package:flutter/material.dart';
import '../../signup.dart';
import '../../login.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  // This widget is the root of our application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      // Application name
      title: 'Registration Form',
      // Application theme data, we can set the colors for the application as
      // you want
      theme: ThemeData(
        primarySwatch: Colors.orange,
      ),
      home: MySignUpPage(),
      routes: {
        'register': (context) => MySignUpPage(),
        'login': (context) => MyLoginPage(),
      },
    );
  }
}

from Login.dart：

import 'package:flutter/material.dart';
import 'package:fluttertoast/fluttertoast.dart';
import '../../authservice.dart';

class MyLoginPage extends StatefulWidget {
  @override
  LoginForm createState() {
    return LoginForm();
  }
}

class LoginForm extends State<MyLoginPage> {
  final formKey = GlobalKey<FormState>();
  final passwordController = TextEditingController();
  final confirmpasswordController = TextEditingController();
  var name, password, token;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        // The title text which will be shown on the action bar
        title: Text('Registration'),
      ),
      body: Container(
        padding: EdgeInsets.symmetric(
          vertical: 50.0,
          horizontal: 10.0,
        ),
        decoration: BoxDecoration(
          image: DecorationImage(
            image: AssetImage("images/signupimg.jpg"),
            fit: BoxFit.cover,
          ),
        ),
        child: Form(
          key: formKey,
          child: Column(
            children: <Widget>[
              Container(
                  margin: EdgeInsets.all(2),
                  padding: const EdgeInsets.all(8.0),
                  child: const Align(
                      alignment: Alignment.topLeft,
                      child: Text(
                        'Login',
                        style: TextStyle(
                          fontSize: 40,
                          fontWeight: FontWeight.bold,
                        ),
                        textAlign: TextAlign.left,
                      ))),
              Container(
                margin: EdgeInsets.all(2),
                padding: const EdgeInsets.all(8.0),
                child: TextFormField(
                  autovalidateMode: AutovalidateMode.onUserInteraction,
                  validator: (String? val) {
                    if (val == null || val.isEmpty) {
                      return 'Username cannot be empty';
                    } else {
                      name = val;
                    }
                    return null;
                  },
                  decoration: InputDecoration(
                      filled: true, //<-- SEE HERE
                      fillColor: Colors.white,
                      labelText: 'Username',
                      hintText: 'Your Username',
                      border: OutlineInputBorder(
                        borderRadius: BorderRadius.circular(20.0),
                      )),
                ),
              ),
              Container(
                margin: EdgeInsets.all(2),
                padding: const EdgeInsets.all(8.0),
                child: TextFormField(
                  obscureText: true,
                  controller: passwordController,
                  autovalidateMode: AutovalidateMode.onUserInteraction,
                  validator: (String? val) {
                    if (val == null || val.isEmpty) {
                      return 'Password cannot be empty';
                    } else if (val.length < 6) {
                      return 'Password must be at least 6 characters long.';
                    } else {
                      password = val;
                    }
                    return null;
                  },
                  decoration: InputDecoration(
                      filled: true, //<-- SEE HERE
                      fillColor: Colors.white,
                      labelText: 'Password',
                      hintText: 'Your password',
                      border: OutlineInputBorder(
                        borderRadius: BorderRadius.circular(20.0),
                      )),
                ),
              ),
              Container(
                margin: EdgeInsets.all(10),
                padding: EdgeInsets.all(10),
                child: ElevatedButton(
                  child: Text(
                    "Login",
                    style: TextStyle(fontSize: 20),
                  ),
                  onPressed: () {
                    AuthService().login(name, password).then((val) {
                      if (val.data['success'] != null) {
                        token = val.data!['token'];
                        Fluttertoast.showToast(
                          msg: 'Authenticated',
                          toastLength: Toast.LENGTH_SHORT,
                          gravity: ToastGravity.BOTTOM,
                          backgroundColor: Colors.white,
                          textColor: Colors.white,
                          fontSize: 16.0,
                        );
                      }
                    });
                    if (formKey.currentState!.validate()) {
                      debugPrint('All validations passed');
                    }
                    style:
                    ElevatedButton.styleFrom(
                      primary: Colors.orange, // Background color
                      onPrimary: Colors.black, //Text Color (Foreground color)
                      elevation: 3, //elevation of button
                      shape: RoundedRectangleBorder(
                          //to set border radius to button
                          borderRadius: BorderRadius.circular(35)),
                    );
                  },
                ),
              ),
              Container(
                margin: EdgeInsets.all(10),
                child: Text(
                  "Don't have an account?",
                  style: TextStyle(fontSize: 16),
                ),
              ),
              Container(
                  child: TextButton(
                      onPressed: () {
                        Navigator.pushNamed(context, 'register');
                      },
                      child: Text("Sign up",
                          style: TextStyle(
                            decoration: TextDecoration.underline,
                            fontSize: 18,
                            color: Colors.orange,
                          ))))
            ],
          ),
        ),
      ),
    );
  }
}

from Authservice.dart 我的服务器）：

import 'package:dio/dio.dart';
import 'package:flutter/material.dart';
import 'package:flutter/cupertino.dart';
import 'package:fluttertoast/fluttertoast.dart';

class AuthService {
  Dio dio = new Dio();
  login(String name, String password) async {
    try {
      return await dio.post(
          'https://flutter-collegeapp.herokuapp.com/authenticate',
          data: {"name": name, "password": password},
          options: Options(contentType: Headers.formUrlEncodedContentType));
    } on DioError catch (e) {
      Fluttertoast.showToast(
        msg: (e.response?.data['msg']),
        toastLength: Toast.LENGTH_SHORT,
        gravity: ToastGravity.BOTTOM,
        backgroundColor: Colors.white,
        textColor: Colors.orange[900],
        fontSize: 16.0,
      );
    }
  }
}

我遵循了本YouTube教程来编写此代码（由于Flutter中的新更新而进行了一些更改）

https://www.youtube.com/watch?v=2d_76lkyf1c&； t = 4903S

有人可以告诉我有什么问题，为什么发生此错误？