Undo Beam Properties change causes crash

1. Open score .
2. Drag-select the eighth notes and rests.
3. Open Beam Properties.
4. Double-click 'middle of beam'.
5. Undo.

Result: Crash.

Using MuseScore 1.0. In 2.0 nightly build (4200), it happens after step 4 - Mac 10.4.11.

Reported by schepers