Crash on changing instrument (from 2-3 channels to 1 channel)

Commit 4e426c4.

Reproduce:
1. Create a new score for violin, put some notes on it
2. Put a staff-text somewhere, make it "pizzicato" or "tremolo" for all voices.
3. Add a Instrument Change element after staff-text and set it to "piano".

Expected behaviour: notes after Instrument Change should sound like a piano.
Actual behaviour: Crash.