在颤振上验证 firebase 时出现问题

Question

我是第一个学习flutter的人。 但是我遇到了一个问题。 我正在测试认证。但是，会出现“createUserWithEmailAndPassword”为空的错误。 我需要有人来帮助我。 我复制了github中的示例。但是，出现了问题。我尝试了很多方法，但找不到解决方案。

import 'package:flutter/material.dart';
import 'package:firebase_auth/firebase_auth.dart';
import 'package:flutter_signin_button/button_builder.dart';

final FirebaseAuth _auth = FirebaseAuth.instance;

/// Entrypoint example for registering via Email/Password.
class RegisterPage extends StatefulWidget {
  /// The page title.
  final String title = 'Registration11';

  @override
  State<StatefulWidget> createState() => _RegisterPageState();
}

class _RegisterPageState extends State<RegisterPage> {
  final GlobalKey<FormState> _formKey = GlobalKey<FormState>();
  final TextEditingController _emailController = TextEditingController();
  final TextEditingController _passwordController = TextEditingController();

  bool _success;
  String _userEmail = '';

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Form(
          key: _formKey,
          child: Card(
            child: Padding(
              padding: EdgeInsets.all(16),
              child: Column(
                crossAxisAlignment: CrossAxisAlignment.start,
                children: <Widget>[
                  TextFormField(
                    controller: _emailController,
                    decoration: const InputDecoration(labelText: 'Email'),
                    validator: (String value) {
                      if (value.isEmpty) {
                        return 'Please enter some text';
                      }
                      return null;
                    },
                  ),
                  TextFormField(
                    controller: _passwordController,
                    decoration: const InputDecoration(labelText: 'Password'),
                    validator: (String value) {
                      if (value.isEmpty) {
                        return 'Please enter some text';
                      }
                      return null;
                    },
                    obscureText: true,
                  ),
                  Container(
                    padding: const EdgeInsets.symmetric(vertical: 16.0),
                    alignment: Alignment.center,
                    child: SignInButtonBuilder(
                      icon: Icons.person_add,
                      backgroundColor: Colors.blueGrey,
                      onPressed: () async {
                        if (_formKey.currentState.validate()) {
                          _register();
                        }
                      },
                      text: 'Register',
                    ),
                  ),
                  Container(
                    alignment: Alignment.center,
                    child: Text(_success == null
                        ? ''
                        : (_success
                        ? 'Successfully registered ' + _userEmail
                        : 'Registration failed')),
                  )
                ],
              ),
            ),
          )),
    );
  }

  @override
  void dispose() {
    // Clean up the controller when the Widget is disposed
    _emailController.dispose();
    _passwordController.dispose();
    super.dispose();
  }

  // Example code for registration.
  void _register() async {
    final User user = (await _auth.createUserWithEmailAndPassword(
      email: _emailController.text,
      password: _passwordController.text,
    ))
        .user;
    if (user != null) {
      setState(() {
        _success = true;
        _userEmail = user.email;
      });
    } else {
      _success = false;
    }
  }
}
---------------------------------------
E/flutter (  843): [ERROR:flutter/lib/ui/ui_dart_state.cc(177)] Unhandled Exception: NoSuchMethodError: The method 'createUserWithEmailAndPassword' was called on null.
E/flutter (  843): Receiver: null
E/flutter (  843): Tried calling: createUserWithEmailAndPassword(email: "firebasetest@gmail.com", password: "123456")