fix #52506: Split Staff on particular slur crashes MuseScore

[lasconic]

In checkSpanner, I make sure that we don't have slurs ending on a rest. The only case where it should happen is during note entry. So when exiting note entry, I delete the pending slur... I know it's a bit weird to do this at this place, so I made a PR. Comments welcome.

[MarcSabatella]

The code makes sense to me, but - I don't think disallowing slurs to rests is a good idea. I suspect a lot of people rely on this to have one-sided slurs that are really "fake" one-sided ties (like into voltas, etc). Or, in older scores (even newer ones), people might be using one-sided slurs to fake falls / scoops etc. Also, the "L.V." / "let ring" indication for stringed instruments?

I guess the crash was because upon moving the note to another staff, the slur was not connected well? Wouldn't it be better to fix the slur rather than delete it? Or, delete it explicitly during the split operation, not as something checkSpanner would do all the time. Not that I know where/how to do that.

[heuchi]

I get a crash when I press 's' on a note that is followed by a rest, when not in note entry mode.

[lasconic]

OK. Thank you both for the feedback. I will try another approach.

[Jojo-Schmitz]

@wschweer fixed it differently in e384f62