广告没有展示，并且chewie播放器没有布置在后退按钮上

Question

我正在实施chewie视频播放器，并在播放器的后退按钮上投放广告。通过将Material App小部件实现为MyApp类，视频播放器可以正常运行，但是广告消失了；另一方面，当我从MyApp屏幕上删除Material App时，广告可以正常运行，但是视频播放器不放置在后退按钮上。

//import 'package:chewie_tutorial/chewie_list_item.dart';
import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
import 'chewie_list_item.dart';
import 'package:firebase_admob/firebase_admob.dart';
import 'package:video_player/video_player.dart';


    class MyApp extends StatelessWidget {
      @override
      Widget build(BuildContext context) {
        Orientatoin();
        return MaterialApp(
          home: MYHomePage(),
        );
      }
    }
    int counter=0;
    class MYHomePage extends StatelessWidget {

      static final MobileAdTargetingInfo targetingInfo = MobileAdTargetingInfo(
        keywords: ['Games', 'Puzzles'],
      );

      BannerAd bannerAd;
      InterstitialAd interstitialAd;
      RewardedVideoAd rewardedVideoAd;

      BannerAd buildBanner() {
        return BannerAd(
            adUnitId: BannerAd.testAdUnitId,
            size: AdSize.banner,
            listener: (MobileAdEvent event) {
              print(event);
            });
      }

      InterstitialAd buildInterstitial() {
        return InterstitialAd(
            adUnitId: InterstitialAd.testAdUnitId,
            targetingInfo: targetingInfo,
            listener: (MobileAdEvent event) {
              if (event == MobileAdEvent.failedToLoad) {
                interstitialAd..load();
              } else if (event == MobileAdEvent.closed) {
                interstitialAd = buildInterstitial()..load();
              }
              print(event);
            });
      }

      @override
      Widget build(BuildContext context) {
        FirebaseAdMob.instance.initialize(appId: FirebaseAdMob.testAppId);
        bannerAd = buildBanner()..load();
        interstitialAd = buildInterstitial()..load();

        Future<bool> _onBackPressed() {
          if(counter<1){
            interstitialAd
              ..load()
              ..show();
            counter++;
          }
          else{
            bannerAd.dispose();

            Navigator.pop(context, true);
          }

        }
        return WillPopScope(
          child:Scaffold(
            body: ListView(
              children: <Widget>[

                ChewieListItem(
                  videoPlayerController: VideoPlayerController.network(
                    'https://bk7l2g37dx53-hls-live.5centscdn.com/adom/5b97b7d64a64c7cd54c18b07e93570a1.sdp/index.m3u8',
                  ),
                ),

              ],
            ),
          ),
          onWillPop: _onBackPressed,
        );
      }
    }
    void Orientatoin() {
      SystemChrome.setPreferredOrientations(
          [DeviceOrientation.landscapeLeft, DeviceOrientation.landscapeRight]);
    }