Xamarin.iOS — экспорт асинхронно завершается с ошибкой

Во-первых, я должен упомянуть, что я новичок в этом (iOS и Xamarin). Я пытаюсь поставить водяной знак на видео, используя изображение и текст.

Я перенес большую часть кода по ссылке: https://stackoverflow.com/a/22016800/5275669.

Весь код вставлен сюда:

try {

            var videoAsset = AVUrlAsset.FromUrl (new NSUrl (filepath, false)) as AVUrlAsset;
            AVMutableComposition mixComposition = AVMutableComposition.Create ();
            var compositionVideoTracks = mixComposition.AddMutableTrack (AVMediaType.Video, 0);
            AVAssetTrack clipVideoTrack = videoAsset.TracksWithMediaType (AVMediaType.Video) [0];
            var compositionAudioTrack = mixComposition.AddMutableTrack (AVMediaType.Audio, 0);
            AVAssetTrack clipAudioTrack = videoAsset.TracksWithMediaType (AVMediaType.Audio) [0];

            NSError error;
            CMTimeRange timeRangeInAsset = new CMTimeRange ();
            timeRangeInAsset.Start = CMTime.Zero;
            timeRangeInAsset.Duration = videoAsset.Duration;
            compositionVideoTracks.InsertTimeRange (timeRangeInAsset, clipVideoTrack, CMTime.Zero, out error);
            compositionVideoTracks.InsertTimeRange (timeRangeInAsset, clipAudioTrack, CMTime.Zero, out error);
            compositionVideoTracks.PreferredTransform = clipVideoTrack.PreferredTransform;

            CGSize sizeOfVideo = videoAsset.NaturalSize;

            CATextLayer textOfvideo = (CATextLayer)CATextLayer.Create ();
            textOfvideo.String = String.Format ("{0} {1}", DateTime.Now.ToLongTimeString (), "Test app");
            textOfvideo.SetFont (CGFont.CreateWithFontName ("Helvetica"));
            textOfvideo.FontSize = 50;
            textOfvideo.AlignmentMode = CATextLayer.AlignmentCenter;
            textOfvideo.Frame = new CGRect (0, 0, sizeOfVideo.Width, sizeOfVideo.Height / 6);
            textOfvideo.ForegroundColor = new CGColor (255, 0, 0);

            UIImage myImage = UIImage.FromFile ("Icon-Small.png");
            CALayer layerCa = CALayer.Create ();
            layerCa.Contents = myImage.CGImage;
            layerCa.Frame = new CGRect (0, 0, 100, 100);
            layerCa.Opacity = 0.65F;

            CALayer optionalLayer = CALayer.Create ();
            optionalLayer.AddSublayer (textOfvideo);
            optionalLayer.Frame = new CGRect (0, 0, sizeOfVideo.Width, sizeOfVideo.Height);
            optionalLayer.MasksToBounds = true;

            CALayer parentLayer = CALayer.Create ();
            CALayer videoLayer = CALayer.Create ();
            parentLayer.Frame = new CGRect (0, 0, sizeOfVideo.Width, sizeOfVideo.Height);
            videoLayer.Frame = new CGRect (0, 0, sizeOfVideo.Width, sizeOfVideo.Height);
            parentLayer.AddSublayer (videoLayer);
            parentLayer.AddSublayer (layerCa);
            parentLayer.AddSublayer (textOfvideo);

            AVMutableVideoComposition videoComposition = AVMutableVideoComposition.Create ();
            videoComposition.RenderSize = sizeOfVideo;
            videoComposition.FrameDuration = new CMTime (1, 30);
            videoComposition.AnimationTool = AVVideoCompositionCoreAnimationTool.FromLayer (videoLayer, parentLayer);

            AVMutableVideoCompositionInstruction instruction = AVMutableVideoCompositionInstruction.Create () as AVMutableVideoCompositionInstruction;
            CMTimeRange timeRangeInstruction = new CMTimeRange ();
            timeRangeInstruction.Start = CMTime.Zero;
            timeRangeInstruction.Duration = mixComposition.Duration;
            instruction.TimeRange = timeRangeInstruction;
            AVAssetTrack videoTrack = mixComposition.TracksWithMediaType (AVMediaType.Video) [0];

            AVMutableVideoCompositionLayerInstruction layerInstruction = AVMutableVideoCompositionLayerInstruction.FromAssetTrack (videoTrack);
            instruction.LayerInstructions = new AVVideoCompositionLayerInstruction[] { layerInstruction };
            List<AVVideoCompositionInstruction> instructions = new List<AVVideoCompositionInstruction> ();
            instructions.Add (instruction);
            videoComposition.Instructions = instructions.ToArray ();

            var exportSession = new AVAssetExportSession (mixComposition, AVAssetExportSession.PresetMediumQuality);
            exportSession.VideoComposition = videoComposition;

            Console.WriteLine ("Original path is {0}", filepath);
            string newFileName = Path.GetFileName (filepath);
            newFileName = newFileName.Replace (".mp4", "_wm.mp4");
            string directoryName = Path.GetDirectoryName (filepath);
            string videoOutFilePath = Path.Combine (directoryName, newFileName);
            Console.WriteLine ("New path is {0}", videoOutFilePath);

            exportSession.OutputFileType = AVFileType.Mpeg4;
            exportSession.OutputUrl = NSUrl.FromFilename (videoOutFilePath);
            exportSession.ShouldOptimizeForNetworkUse = true;
            exportSession.ExportAsynchronously (() => {
                AVAssetExportSessionStatus status = exportSession.Status;
                Console.WriteLine ("Done with handler. Status: " + status.ToString ());
                switch (status) {
                case AVAssetExportSessionStatus.Completed:
                    Console.WriteLine ("Sucessfully Completed");
                    if (File.Exists (videoOutFilePath)) {
                        Console.WriteLine ("Created!!");
                    } else
                        Console.WriteLine ("Failed");
                    break;
                case AVAssetExportSessionStatus.Cancelled:
                    break;
                case AVAssetExportSessionStatus.Exporting:
                    break;
                case AVAssetExportSessionStatus.Failed:
                    Console.WriteLine ("Task failed => {0}", exportSession.Error);
                    Console.WriteLine (exportSession.Error.Description);
                    break;
                case AVAssetExportSessionStatus.Unknown:
                    break;
                case AVAssetExportSessionStatus.Waiting:
                    break;
                default:
                    break;
                }
            });

            if (File.Exists (videoOutFilePath))
                return videoOutFilePath;
        } catch (Exception ex) {
            Console.WriteLine ("Error occured : {0}", ex.Message);
        }

Я продолжаю получать следующую ошибку: Сбой задачи => Невозможно завершить экспорт Ошибка домена = AVFoundationErrorDomain Code =-11820 «Невозможно завершить экспорт» UserInfo = 0x18896070 {NSLocalizedRecoverySuggestion = Попробуйте выполнить экспорт еще раз., NSLocalizedDescription = Невозможно завершить экспорт}

Если я заменю это

var exportSession = new AVAssetExportSession (mixComposition, AVAssetExportSession.PresetMediumQuality);

с участием

var exportSession = new AVAssetExportSession (videoAsset, AVAssetExportSession.PresetMediumQuality);

он работает нормально, но без водяного знака

Может кто-нибудь помочь с этим??