google_mobile_ads occurs an error when banner widget displayed | flutter

Question

I'm trying to display banner ad in my app on Android. But an error occurs when AdWidget() called.

    throw FlutterError.fromParts(<DiagnosticsNode>[
      ...information,
      DiagnosticsProperty<BoxConstraints>('The constraints that applied to the $runtimeType were', constraints, style: DiagnosticsTreeStyle.errorProperty),
      DiagnosticsProperty<Size>('The exact size it was given was', _size, style: DiagnosticsTreeStyle.errorProperty),
      ErrorHint('See https://flutter.dev/docs/development/ui/layout/box-constraints for more information.'),
    ]);

But I couldn't find the error in log. VScode just showed me this box.dart marked line.

What I Did

Edit AndroidManifest.xml

Edited AndroidManifest.xml in android/app/src/main.

<application>
      .....

    <meta-data
        android:name="com.google.android.gms.ads.APPLICATION_ID"
        android:value="***********************************"/>

      ....
</application>

Initialize Admob

void main() async{

    WidgetsFlutterBinding.ensureInitialized();
    await MobileAds.instance.initialize();

}

Create instance in build()

// In class extends StatefulWidget's State

@override
Widget build(BuildContext context) {
  BannerAd bannerAd = BannerAd(
    adUnitId: getBannerAdUnitId(),
    size: AdSize.banner,
    request: const AdRequest(),
    listener: const BannerAdListener(),
  );
  bannerAd.load();
.....

Call AdWidget(bannerAd)

return Center(
  child: Column( 
    children: [
      AdWidget(bannerAd),
      ..........
]);

When I remove AdWidget(bannerAd) from the code, no error has shown.