如何将Admob的广告添加到Flutter widgets树？

dependencies:
  flutter:
    sdk: flutter
  google_mobile_ads: ^1.0.0

<manifest>
    <application>
        <!-- 确保替换为你的AdMob app ID -->
        <meta-data
            android:name="com.google.android.gms.ads.APPLICATION_ID"
            android:value="ca-app-pub-xxxxxxxxxxxxxxxx~yyyyyyyyyy"/>
    </application>
</manifest>

<key>GADApplicationIdentifier</key>
<string>ca-app-pub-xxxxxxxxxxxxxxxx~yyyyyyyyyy</string>
<key>SKAdNetworkItems</key>
<array>
    <dict>
        <key>SKAdNetworkIdentifier</key>
        <string>cstr6suwn9.skadnetwork</string>
    </dict>
</array>

import 'package:google_mobile_ads/google_mobile_ads.dart';

void main() {
  WidgetsFlutterBinding.ensureInitialized();
  MobileAds.instance.initialize();
  runApp(MyApp());
}

BannerAd myBanner = BannerAd(
  adUnitId: 'ca-app-pub-3940256099942544/6300978111', // 测试ID，替换为你的广告单元ID
  size: AdSize.banner,
  request: AdRequest(),
  listener: BannerAdListener(
    onAdLoaded: (Ad ad) => print('Ad loaded.'),
    onAdFailedToLoad: (Ad ad, LoadAdError error) {
      ad.dispose();
      print('Ad failed to load: $error');
    },
  ),
);

myBanner.load();

@override
Widget build(BuildContext context) {
  return Scaffold(
    appBar: AppBar(
      title: Text('AdMob Example'),
    ),
    body: Column(
      children: <Widget>[
        Expanded(
          child: Center(
            child: Text('Hello, Flutter!'),
          ),
        ),
        Container(
          alignment: Alignment.center,
          child: AdWidget(ad: myBanner),
          width: myBanner.size.width.toDouble(),
          height: myBanner.size.height.toDouble(),
        ),
      ],
    ),
  );
}