Instrument changing in the parts causes crash

For checking and understand what is going on, we need you to attach the score where you observe this.

Ok, understood now.

Steps:

1) SATB template
2) Create parts (New all -> Ok)
3) Toggle in Soprano Part
4) Right- click -> Staff properties -> Change instrument -> Select e.g. Violin -> Ok

---> Crash

Possibly relates to #286842: Assertion failure on adding instrument change in score with parts. BTW, in master, it doesn't crash, but it doesn't actually accomplish anything, either.

The fix for the crash itself is included in this PR (namely, this commit) but surprisingly I see the relevant part of the code unchanged in 3.0.4 and 3.0.5 versions. It looks like only some of the commits from that PR were picked to these versions.

Concerning the instrument change in parts having no effect, the reason for it looks clear: the fix makes the instruments information from master score have more priority than that from the parts. It looks like the fix for this issue could be reworking the ChangePart undo command which does the actual instrument change job to change instruments in master score and parts synchronously. It will hardly help with #286842: Assertion failure on adding instrument change in score with parts directly but maybe something similar could be used there too.

Given it doesn't work to have different transpositions between score and part (we would need more than just two tpc's for this), I do think linking the instrument changes make sense, but we'd also want to think about linking transposition for the staff itself see #62416: Changes to staff transposition (and other properties) not reflected in linked parts

The crash is fixed in version 3.1 (as stated above, in this commit).

Automatically closed -- issue fixed for 2 weeks with no activity.