After switching from one project to another, when I switched back to the first project, the timeline was empty. Everything returned to the timeline when I chose "undo" from the edit menu (and it 'undid' the last edit I made in the project). So I wasn't too concerned. But now, after closing the application and opening it again, the timeline is once again empty. And I can't use the undo trick, because there is nothing to undo. Help!!