Why does Visual Studio interrupt a code that is tested to be working?

I am working on a UWP app using Visual Studio 2017. In the app there is a task that takes a BitmapImage and apply a color to it. The task exists on both the MainPage.Xaml.cs and a background task called BackgroundTask.cs.

private static async Task<WriteableBitmap> GetImageFile(Uri fileUri)
    {
        StorageFile imageFile = await StorageFile.GetFileFromApplicationUriAsync(fileUri);

        WriteableBitmap writeableBitmap = null;
        using (IRandomAccessStream imageStream = await imageFile.OpenReadAsync())
        {
            BitmapDecoder bitmapDecoder = await BitmapDecoder.CreateAsync(imageStream);

            BitmapTransform dummyTransform = new BitmapTransform();
            PixelDataProvider pixelDataProvider =
               await bitmapDecoder.GetPixelDataAsync(BitmapPixelFormat.Bgra8,
               BitmapAlphaMode.Premultiplied, dummyTransform,
               ExifOrientationMode.RespectExifOrientation,
               ColorManagementMode.ColorManageToSRgb);
            byte[] pixelData = pixelDataProvider.DetachPixelData();

            writeableBitmap = new WriteableBitmap(
               (int)bitmapDecoder.OrientedPixelWidth,
               (int)bitmapDecoder.OrientedPixelHeight);
            using (Stream pixelStream = writeableBitmap.PixelBuffer.AsStream())
            {
                await pixelStream.WriteAsync(pixelData, 0, pixelData.Length);
            }
        }
        return writeableBitmap;
    }

The code works perfectly on the MainPage. However, when trying to run in the background, Visual Studio starts an exception with no clear reasons at a much earlier stage of the code:

System.Exception: 'The application called an interface that was marshalled for a different thread. (Exception from HRESULT: 0x8001010E (RPC_E_WRONG_THREAD))'

The code is interrupted at this stage:

byte[] pixelData = pixelDataProvider.DetachPixelData();
writeableBitmap = new WriteableBitmap(
           (int)bitmapDecoder.OrientedPixelWidth,
           (int)bitmapDecoder.OrientedPixelHeight);
using (Stream pixelStream = writeableBitmap.PixelBuffer.AsStream())
  {
    await pixelStream.WriteAsync(pixelData, 0, pixelData.Length);
  }

Using this solution gives me this error:

System.Runtime.InteropServices.COMException: 'A method was called at an unexpected time.

Could not create a new view because the main window has not yet been created'

the background task does not change the UI and do not need a window to be created! I don't know the reason behind this interruption and I don't know what information is needed to fix it. Does anyone have any experience with this?