We are working on finding the cause(s) of this issue and hope to have them resolved completely in the future. In the meantime, it seems that in most cases, you can recover by closing the app (double-click on the Home button and swipe up on it's screenshot) then start it again. You should come back right where you left off and the preview should return.
If you find a situation where this doesn't recover the preview, try removing or adding a single clip in the timeline and see if that recovers your preview.
Please contact us at firstname.lastname@example.org
if neither of these recover you preview, or if you find that nearly any edit within a project loses the preview again and again.