我正在使用Mockitou包来测试我的MessageBloc和我的消息应用程序在Flutter中的逻辑，但我在标题中提到的所有测试中都有一个共同的问题（返回类型'Null'不是'Future'，这是闭包上下文所要求的。）

import 'package:chat/chat.dart';
import 'package:flutter_newapp/src/blocs/message/message_bloc.dart';
import 'package:flutter_newapp/src/blocs/message/message_event.dart';
import 'package:flutter_newapp/src/blocs/message/message_state.dart';
import 'package:flutter_test/flutter_test.dart';
import 'package:mockito/mockito.dart';

class FakeMessageService extends Mock implements IMessageService {}

void main() {
  late MessageBloc sut;
  late IMessageService messageService;
  User user;

  setUp(() {
    messageService = FakeMessageService();
    user = User(
        username: 'test', photoUrl: '', active: true, lastSeen: DateTime.now());
    sut = MessageBloc(messageService);
  });

  tearDown(() => sut.close());

  test('it should emit initial state before Subscription',
      () => expect(sut.state, MessageInitial()));

  test('should emit message sent state when message is sent', () {
    final message = Message(
        from: '1234',
        to: '123',
        timestamp: DateTime.now(),
        contents: 'Hellllo');
    when(messageService.send([message])).thenAnswer((_) async => null); // Here is the Error
    sut.add(MessageEvent.onMessageSent([message]));
    expectLater(sut.stream, emits(MessageState.sent(message)));
  });
}

因此，如果任何人有一个解决方案，我将非常感激
我必须重写IMessageService的方法还是什么？我认为这就是问题所在