I can save the UserControl to image in WPF, but the pictures is black

I can save all the UserControl to image. But all the pictures is dark when I click save button in the first time except current UserControl, That's ok when I click save button again.

This is my C# code:

public static void SaveView(UserControl viewName, int dpi, string fileName, string destFolder)
        {
            Rect bounds = VisualTreeHelper.GetDescendantBounds(viewName);

            int width = (int)viewName.RenderSize.Width;
            int height = (int)viewName.RenderSize.Height;

            if (width == 0 && height == 0)
            {
                width = 1920;
                height = 1080;
            }

            RenderTargetBitmap rtb = new RenderTargetBitmap(
                    width,
                    height,
                    dpi,
                    dpi,
                    PixelFormats.Pbgra32
                );


            DrawingVisual dv = new DrawingVisual();

            using (DrawingContext ctx = dv.RenderOpen())
            {
                VisualBrush vb = new VisualBrush(viewName);
                ctx.DrawRectangle(vb, null, new Rect(new Point(), bounds.Size));
            }

            rtb.Render(viewName);

            try
            {
                PngBitmapEncoder jpgEncoder = new PngBitmapEncoder();
                jpgEncoder.Frames.Add(BitmapFrame.Create(rtb));

                Byte[] _imageArray;

                using (MemoryStream outputStream = new MemoryStream())
                {
                    jpgEncoder.Save(outputStream);
                    _imageArray = outputStream.ToArray();
                }

                //Try Find Save Path, if doesn't exists, create it.
                if (Directory.Exists(destFolder) == false)
                    Directory.CreateDirectory(destFolder);

                FileStream fileStream = new FileStream(Path.Combine(destFolder, fileName), FileMode.Create, System.IO.FileAccess.Write);

                fileStream.Write(_imageArray, 0, _imageArray.Length);
                fileStream.Close();
            }
            catch (Exception e)
            {
                Log4Net.Instance.Info(System.Reflection.MethodInfo.GetCurrentMethod().ToString());
                Log4Net.Instance.Info("Exception Generate Screenshot: " + fileName);
                Log4Net.Instance.Info(e.StackTrace.ToString());
                Debug.WriteLine(e.ToString());
            }
        }