[Trunk] Quitting, then cancelling to an unsaved score quits MuseScore

1. Open score.
2. Change something (causing an impending save).
3. Quit.
4. Cancel.

Expected Result: Score remains open.
Actual result: Program quits. Re-launching it reveals it quit unexpectedly.

Discussion: Marked as critical as you lose your work.

Using MuseScore 2.0 Nightly Build (4801) - Mac 10.6.8.