onAdError callback not called when there is no ad

Hi, i use unity native android library to show popup video ads, but sometimes
onUnityAdsError event not called.
I build github sample project and problem exists.
This is my code :

Logger.e(TAG, "Request Unity popup ads . . .");

    UnityAds.initialize(context, UNITY_KEY, new IUnityAdsListener() {

        [USER=530556]@override[/USER]

        public void onUnityAdsReady(final String zoneId) {

            try {

                Logger.d("onUnityAdsReady: " + zoneId);

                Logger.e(TAG, "Unity popup ads Ready");

                switch (zoneId) {

                    case UNITY_POPUP_ZONE_ID:

                        if (listener != null)

                            listener.initializeFinished();

                        break;

                }

            } catch (Exception e) {

                e.printStackTrace();

            }

        }



        [USER=530556]@override[/USER]

        public void onUnityAdsStart(String zoneId) {

            Logger.e(TAG, "Unity popup ads Start");

        }



        [USER=530556]@override[/USER]

        public void onUnityAdsFinish(String zoneId, UnityAds.FinishState result) {

            try {

                Logger.d("onUnityAdsFinish: " + zoneId + " - " + result);

            } catch (Exception e) {

                e.printStackTrace();

            }

        }



        [USER=530556]@override[/USER]

        public void onUnityAdsError(UnityAds.UnityAdsError error, String message) {

            try {

                Logger.e(TAG, "Unity popup ads Error");

            } catch (Exception e) {

                e.printStackTrace();

            }

        }

    }, false);

}