[trunk] Triplet causes crash

Steps to reproduce bug
1. Open Bach-bc2 demo
2. Click on the first eight-note of the trumpet staff
3. Notes > Tuplets > Triplets
4. Click on the first rest inside the triplet to add a note

Expected behavior: Note is added to triplet

Actual behavior: Application crashes

MuseScore version: Nightly r. 3559 trunk

(Operating System: Windows 7)