void onError(AndroidJavaObject ad,
                     AndroidJavaObject error)
        {
            string errorMessage = error.Call <string>("getErrorMessage");

            if (interstitialAd.InterstitialAdDidFailWithError != null)
            {
                interstitialAd.ExecuteOnMainThread(() =>
                {
                    interstitialAd.InterstitialAdDidFailWithError(errorMessage);
                });
            }
        }