Fix #30959: Crash when dragging tremolo from palette

[mathesoncalum]

Resolves: #30959

See comment - data.track is a nidx in this case so SysStaff evaluates to null

[XiaoMigros]

Why are tremolos able to call Measure::acceptDrop in the first place? They should be caught by the earlier code (Note::acceptDrop and Chord::drop). Either way, EditData::track should be set before being able to reach acceptDrop, which is where I think a fix should investigate. The reason for switching to a stored track as opposed to a position is twofold: On the one hand, it saves calls to pos2measure, but more importantly, it stops dropping/pasting to invisible staves from behaving unpredictably.

[mathesoncalum]

OK thanks for the clarification - I think that makes sense!

[mathesoncalum]

To answer your question about calling acceptDrop with a tremolo - take a look at prepareDropStandardElement and how we populate the "droppable tree". We do this once every time we start a new drag from the palette - we go through all elements on the page and collect those that accept a drop for the given item. Critically we don't just scan through notes/chords, so it is possible that we can end up querying a measure (in which case we should just return false).

[XiaoMigros]

Since 10eadab, maybe that tree iteration can be improved upon, as any notes or rests in a measure will call Measure::acceptDrop automatically.

[mathesoncalum]

Ok, I'm just going to fix the crash in this PR but you raise an interesting point about that optimisation @XiaoMigros. I've stuck that on my todo list.

Thanks for your help on this one!

[zacjansheski]

Tested on MacOS 15, Windows 11, Ubuntu 22.04.3. Approved
#30959 FIXED