Android Admob Adreward无法加载，错误代码为0

当我尝试加载广告时，有时会显示错误代码0

rewardedAd = new RewardedAd(getActivity(), "ca-app-pub-3940256099942544/5224354917"); 
RewardedAdLoadCallback adLoadCallback = new RewardedAdLoadCallback() {
        @Override
        public void onRewardedAdLoaded() {
            // Ad successfully loaded
        }

        @Override
        public void onRewardedAdFailedToLoad(int errorCode) {
            // Ad failed to load, I got error code = 0 here
        }
    };

在这篇文章中，作者说：

错误代码0：错误\u代码\u内部错误这表示 发生在内部；例如，接收到无效的响应 从广告服务器

新创建的广告单元最多需要2小时才能激活（启动 时间）。来自新创建的广告单位的任何广告请求也将收到 在Ad请求准备就绪之前，响应Ad请求时出现错误代码0 现场广告

我不认为这是问题所在，因为我只使用了测试广告id

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任何帮助都将不胜感激

以下是在应用程序中正确实施奖励广告的步骤

private RewardedVideoAd mRewardedVideoAd;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
     mRewardedVideoAd = MobileAds.getRewardedVideoAdInstance(this);
     loadRewardedVideoAd();
}

public void loadRewardedVideoAd() {

    mRewardedVideoAd.setRewardedVideoAdListener(new RewardedVideoAdListener() {
        @Override
        public void onRewardedVideoAdLoaded() {

        }

        @Override
        public void onRewardedVideoAdOpened() {
        }

        @Override
        public void onRewardedVideoStarted() {
        }

        @Override
        public void onRewardedVideoAdClosed() {
          loadRewardedVideoAd();
        }

        @Override
        public void onRewarded(RewardItem rewardItem) {
        }

        @Override
        public void onRewardedVideoAdLeftApplication() {
        }

        @Override
        public void onRewardedVideoAdFailedToLoad(int i) {

        }

        @Override
        public void onRewardedVideoCompleted() {


        }
    });

    mRewardedVideoAd.loadAd("ca-app-pub-3940256099942544/5224354917",
            new AdRequest.Builder().build());
}

public void showRewardedAds() {
    if (mRewardedVideoAd != null && mRewardedVideoAd.isLoaded()) {
        mRewardedVideoAd.show();
    }  else {

      loadRewardedVideoAd();
    }
}

 @Override
public void onDestroy() {
    if (mRewardedVideoAd != null) {
        mRewardedVideoAd.destroy(this);
    }
    super.onDestroy();
}