如何在flutter中实现"open app“admob广告？

import 'package:flutter/material.dart';
import 'package:firebase_admob/firebase_admob.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'AdMob Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  final String appId = 'YOUR_ADMOB_APP_ID';
  final String adUnitId = 'YOUR_OPEN_APP_AD_UNIT_ID';

  @override
  void initState() {
    super.initState();
    FirebaseAdMob.instance.initialize(appId: appId);
    _loadAd();
  }

  void _loadAd() {
    RewardedVideoAd.instance.load(
      adUnitId: adUnitId,
      targetingInfo: MobileAdTargetingInfo(),
    );
  }

  void _showAd() {
    RewardedVideoAd.instance.show();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('AdMob Demo'),
      ),
      body: Center(
        child: RaisedButton(
          child: Text('Show Ad'),
          onPressed: _showAd,
        ),
      ),
    );
  }
}