code: protected void onAdFailed(final int adIdx, final int errorCode, final Object adPayload) {
          final Context context = mContext.get();
          //context may be null if activity is destroyed
          if(context != null) {
              for (AdmobListener listener : mAdNativeListeners)
                  listener.onAdFailed(adIdx, errorCode, adPayload);
          }
      }
docs: Notifies all registered {@link AdmobListener} on a failed ad.