java.lang.RuntimeException:无法实例化Flutter中的活动

Flutter新创建的应用程序在使用fire base时出错。

我应该如何解决这个问题，因为它要求flutter应用程序的主活动不能转换为android应用程序的活动。

04-05 21:16:07.570 27391-27391/? E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.firestoreflutterchat, PID: 27391
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.firestoreflutterchat/com.example.firestoreflutterchat.MainActivity}: java.lang.ClassCastException: com.example.firestoreflutterchat.MainActivity cannot be cast to android.app.Activity
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2124)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2257)
at android.app.ActivityThread.access$800(ActivityThread.java:139)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1210)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:136)
at android.app.ActivityThread.main(ActivityThread.java:5097)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:515)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:785)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601)
at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.ClassCastException: com.example.firestoreflutterchat.MainActivity cannot be cast to android.app.Activity
at android.app.Instrumentation.newActivity(Instrumentation.java:1084)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2115)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2257) 
at android.app.ActivityThread.access$800(ActivityThread.java:139) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1210) 
at android.os.Handler.dispatchMessage(Handler.java:102) 
at android.os.Looper.loop(Looper.java:136) 
at android.app.ActivityThread.main(ActivityThread.java:5097) 
at java.lang.reflect.Method.invokeNative(Native Method) 
at java.lang.reflect.Method.invoke(Method.java:515) 
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:785) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601) 
at dalvik.system.NativeStart.main(Native Method) 

这是主dart文件，这里没有任何更改。

import 'package:flutter/material.dart';
import 'package:cloud_firestore/cloud_firestore.dart';
void main() {
runApp(MyApp());
}
class MyApp extends StatelessWidget {
// This widget is the root of your application.
@override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
primarySwatch: Colors.blue,
visualDensity: VisualDensity.adaptivePlatformDensity,
),
home: MyHomePage(title: 'Flutter Demo Home Page'),
);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key key, this.title}) : super(key: key);

final String title;
@override
_MyHomePageState createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
int _counter = 0;
void _incrementCounter() {
setState(() {
_counter++;
});
}
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text(widget.title),
),
body: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
Text(
'You have pushed the button this many times:',
),
Text(
'$_counter',
style: Theme.of(context).textTheme.headline4,
),
],
),
),
floatingActionButton: FloatingActionButton(
onPressed: _incrementCounter,
tooltip: 'Increment',
child: Icon(Icons.add),
), 
);
}
}

这是android src主要活动kotlin:

package com.example.firestoreflutterchat
import android.content.Context
//import androidx.multidex.MultiDex
import io.flutter.app.FlutterApplication
import io.flutter.embedding.android.FlutterActivity
class MainActivity : FlutterApplication() {

}

我只打开了一个新的flutter项目，并按照fire-base步骤将依赖项和google-services.json文件添加到项目中。

后来它在里面给出了firebase错误，所以我添加了multidex。

在flutter应用程序和android模块main活动的主dart文件中，没有进行任何更改。

是不是我做错了什么，我是新来的，所以我不理解。