Switching tab and undo redo causes crash

Musescore b5332dd7c6

1. Open a score, any score
2. Create another score, add 4 quarter note in a measure but not the two first one
3. Copy this measure, and paste it in the first measure
4. Select the first measure and insert a measure before it
5. Undo twice
6. Switch to the first score
7. Switch back the the score and Redo twice

Result: Crash

Discussion: If you don't switch to another score, there is no crash. My best guess is that the undo history is broken because the score is relayouted when we switch back to it. Relayouting will call undo() several times (like in addsystemheader) and that could break the history.