how to print on inkCanvas on mvvm?

Question

Hello:
as I can print in mvv m with inkcanvas ? I have this code and I get a failure Object reference not set to an instance of an object.'

this code the PrinterHelper.cs
{

    public  static PrintHelper _printHelper;

    private readonly InkCanvas _inkCanvas;
    private readonly Canvas _printCanvas;

    public PrinterHelper (InkCanvas inkCanvas,Canvas printCanvas)
    {
        _inkCanvas = inkCanvas;

        _printCanvas = printCanvas ;
    }


    //Craete Bitmap from strokes

    public async Task PrintHelper()
    {
        var inkStream = new InMemoryRandomAccessStream();
        await _inkCanvas.InkPresenter.StrokeContainer.SaveAsync(inkStream.GetOutputStreamAt(0));
        var inkBitmap = new BitmapImage();
        await inkBitmap.SetSourceAsync(inkStream);

        //you need to adjust to layout the image properly in the print-page.

        var inkBounds = _inkCanvas.InkPresenter.StrokeContainer.BoundingRect;
        var inkMargin = new Thickness(inkBounds.Left, inkBounds.Top, _inkCanvas.ActualWidth - inkBounds.Right, _inkCanvas.ActualHeight - inkBounds.Bottom);

        //prepare Viebox + image to be printed.

        var inkViewBox = new Viewbox()
        {
            Child = new Image()
            {
                Source = inkBitmap,
                Margin = inkMargin
            },

            Width = _inkCanvas.ActualWidth,
            Height = _inkCanvas.ActualHeight
        };


        _printHelper = new PrintHelper(_printCanvas);
       _printHelper.AddFrameworkElementToPrint(inkViewBox);

        var printHelperOptions = new PrintHelperOptions();

        printHelperOptions.AddDisplayOption(StandardPrintTaskOptions.Copies);
        printHelperOptions.AddDisplayOption(StandardPrintTaskOptions.MediaSize);
        printHelperOptions.AddDisplayOption(StandardPrintTaskOptions.Collation);
        printHelperOptions.AddDisplayOption(StandardPrintTaskOptions.Orientation);

        printHelperOptions.PrintQuality = PrintQuality.Default;
        printHelperOptions.MediaSize = PrintMediaSize.Default;
        printHelperOptions.Collation = PrintCollation.Default;
        printHelperOptions.Collation = PrintCollation.Default;
        printHelperOptions.Orientation = PrintOrientation.Landscape;
    }



    private async void PrintHelper_OnPrintSucceded()
    {
        _printHelper.Dispose();
        var dialog = new MessageDialog("Printing Done");
        await dialog.ShowAsync();
    }

    private async void PrintHelper_OnPrintFailed()
    {
        _printHelper.Dispose();
        var dialog = new MessageDialog("Printing Failed");
        await dialog.ShowAsync();
    }

    public  static  async Task ShowPrintUIAsync()
    {
        var printHelperOptions = new PrintHelperOptions();

        await _printHelper.ShowPrintUIAsync("Printing INKPen", printHelperOptions);
    }

}

}

and buttton private async void PrintButton_Click(object sender, RoutedEventArgs e) { await Components.PrinterHelper.ShowPrintUIAsync(); }