This is currently very easy to reproduce, and makes the release editor practically unusable...
Steps to reproduce:
Pick a release, any release, and try to edit it. Here's one: http://test.musicbrainz.org/release/ade577f6-6087-4a4f-8e87-38b0f8169814/edit
Switch to the tracklist tab, and change something (anything) using either the basic or advanced tracklist editor. Adding a couple random characters to a track name is enough.
Use either the previous/next buttons or one of the tab buttons to try to switch to a different screen.
You will be brought back to the tracklist editor tab, except that the tracklist has now disappeared.
If you attempt to switch editor tabs now, the release editor will crash with a server error.