Buttons missing on save changes dialog when a second window is open

Issue type

UI bug (incorrect info or interface appearance)

Description with steps to reproduce

Master branch

1. Create a new score (e.g. From Template -> General -> Treble clef).
2. Create another score so that another MSS window appears (single-process mode is ON).
3. Go back to the first score and try to close it or to close the entire window.

Result: The "Do you want to save your changes" dialog appears but no buttons appear on it.
If you do the same for the second window, the save changes dialog appears correctly there.
When you close the secod window (closing just the second score is not sufficient), restores proper save changes dialog in first window.

Some errors appear in the logs:
23:01:06.604 | INFO | main_thread | ActionsDispatcher::doDispatch | try call action: file-close
23:01:06.606 | DEBUG | main_thread | | try open uri: muse://interactive/standard, page: {"modal":"","module":"Muse.Interactive","params":{"buttons":[],"contentTitle":"Do you want to save changes to the score ￢ﾀﾜUntitled score￢ﾀﾝ before closing?","customButtons":[{"buttonId":18,"isAccent":false,"isLeftSide":false,"role":9,"text":"Don￢ﾀﾙt save"},{"buttonId":8,"isAccent":false,"isLeftSide":false,"role":9,"text":"Cancel"},{"buttonId":20,"isAccent":true,"isLeftSide":false,"role":9,"text":"Save"}],"defaultButtonId":20,"detailedText":"","dialogTitle":"","text":"Your changes will be lost if you don￢ﾀﾙt save them.","textFormat":2,"type":"WARNING","withDontShowAgainCheckBox":false,"withIcon":true},"path":"StandardDialog","type":2,"uri":"muse://interactive/standard"}
23:01:06.614 | WARN | main_thread | Qt | qrc:/qt/qml/Muse/UiComponents/ButtonBox.qml:85: TypeError: Value is null and could not be converted to an object
23:01:06.617 | WARN | main_thread | Qt | qrc:/qt/qml/Muse/UiComponents/ButtonBox.qml:75: TypeError: Cannot read property 'accentButton' of null
23:01:06.617 | WARN | main_thread | Qt | qrc:/qt/qml/Muse/Interactive/StandardDialogPanel.qml:131: TypeError: Cannot read property 'text' of null

https://github.com/user-attachments/assets/7417c04f-42c5-4a77-b94e-7330abbc7ddf

Supporting files, videos and screenshots

n/a

In which versions of MuseScore Studio is this issue present?

master

Regression

Yes, this works in the latest released version and is broken in the nightly builds

Operating system

Windows 10

Additional context

No response

Checklist

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• I have verified that this issue has not been logged before, by searching the issue tracker for similar issues
• I have attached all requested files and information to this report
• I have attempted to identify the root problem as concisely as possible, and have used minimal reproducible examples where possible