[MU4 Issue] Crash when deleting staff containing system objects and all staves below in existing score with system objects and "Segno" marking

[cbjeukendrup]

To Reproduce Steps to reproduce the behavior:

1. Create a score from a template with system objects, like SATB+Piano or Symphony Orchestra
2. Add a "Segno" marking in some measure
3. Delete the staff that shows the system objects and all staves below if any (so in the case of SATB+Piano, the Piano, and in the case of Symphony Orchestra, the whole strings section) -> instant crash

Additional context Originally reported in https://musescore.org/en/node/341484 I can't reproduce it with a Tempo marking rather than a Segno marker; for the Tempo, instead of a crash, the marking would be duplicated on the top staff. Not yet fixed by https://github.com/musescore/MuseScore/pull/15715