private static void InterstitialDidFailToLoadAdCallback(
            IntPtr interstitialClient, string error)
        {
            InterstitialClient client = IntPtrToInterstitialClient(
                interstitialClient);

            if (client.OnInterstitialFailedToLoad != null)
            {
                AdFailureEventArgs args = new AdFailureEventArgs()
                {
                    Message = error
                };
                client.OnInterstitialFailedToLoad(client, args);
            }
        }