onAdFailedToLoad：尝试集成广告时出现 0 错误

W/flutter ( 2864): onAdFailedToLoad: 0
I/Ads     ( 2864): Ad failed to load : 0
I/flutter ( 2864): BannerAd event is MobileAdEvent.failedToLoad

可能是什么问题呢？

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

const String testDevice = 'Mobile_id';

void main() => runApp(MyApp());

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  MobileAdTargetingInfo targetingInfo;
  BannerAd myBanner;
  @override
  void initState() {
    super.initState();
    targetingInfo = MobileAdTargetingInfo(
      keywords: <String>['flutterio', 'beautiful apps'],
      contentUrl: 'https://flutter.io',
      birthday: DateTime.now(),
      childDirected: false,
      designedForFamilies: false,
      gender: MobileAdGender
          .male, // or MobileAdGender.female, MobileAdGender.unknown
      testDevices: <String>[], // Android emulators are considered test devices
    );
    myBanner = BannerAd(
      // Replace the testAdUnitId with an ad unit id from the AdMob dash.
      // https://developers.google.com/admob/android/test-ads
      // https://developers.google.com/admob/ios/test-ads
      adUnitId: BannerAd.testAdUnitId,
      size: AdSize.smartBanner,
      targetingInfo: targetingInfo,
      listener: (MobileAdEvent event) {
        print("BannerAd event is $event");
      },
    );
  }

  void _show_banner() {
    myBanner
      ..load()
      ..show(
        anchorOffset: 0.0,
        anchorType: AnchorType.bottom,
      );
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      title: 'Title',
      theme: ThemeData(),
      home: Scaffold(
        appBar: AppBar(
          title: Text('data'),
        ),
        body: Center(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: <Widget>[
              RaisedButton(
                onPressed: () => _show_banner(),
                child: Text('Show Banner Ad'),
              )
            ],
          ),
        ),
      ),
    );
  }
}

AndroidManifest.xml在之后还添加了一个标签

<application>

<meta-data
            android:name="com.google.android.gms.ads.APPLICATION_ID"
            android:value="ca-app-pub-3940256099942544~3347511713"/>
</application>