When I use the 'Green Screen' effect to remove a video green screen, and create a video with a transparent background (eg shilhouette dancers), it is fine on time line & clip preview, but the program always subs a black background when viewing sequence preview or exporting the finished MP4 file. In the past I simply removed the green screen background with the green screen effect (chroma key) and exported the transparent file to my computer. Worked fine. Now it appears there is a default color background (black) added to any file exported. What's changed and how to correct this?