Crash with dropping treble clef over bass clef

Reported by octatone at http://musescore.org/en/node/1379 and reproduced:

If you insert two mid-measure clef changes (for example keeping a scalar run within the staff), and you try to change/undo the clef by dragging the original clef from the pallet over the first clef change. Mscore crashes and closes. Does not freeze, just shuts down.