Crash due to no duration selected by default in node entry toolbar

Steps to reproduce.
1. Open up MuseScore, close default score, create one from scratch (if it's not visible, drag it around a bit, this is reported as well).
2. Click the N button (to go in note entry mode). No duration is selected by default. Hover a staff and click as if you want to insert a note.

Actual result: MuseScore crashes.
Expected result: MuseScore should refuse to place a note because no duration was given, but should continue working.

The fix might be to enter an additional "if" clause just for the sake of this situation (no note duration selected), or might ensure that there is no way, ever, to have no note duration selected.

Reproduced using r2830 (I'm *always* on Win XP sp3, 1.80Ghz dual core, 1.8 GB RAM)