Flutter新创建的应用程序在使用Fire Base时出现错误。
我应该如何解决这个问题，因为它是问flutter应用程序的主要活动不能铸造到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，此处不做任何更改。

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),
      ), 
    );
  }
}

这是安卓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的步骤将依赖项和google-services.json文件添加到项目中。
它后来在错误中给出了firebase，所以我添加了multidex。
在flutter应用程序的主dart文件和android模块主活动中，未进行任何变更。
是不是我做错了什么，我是新来的，所以我不明白。