Possible crash in `AbstractWorldTool::undoStack` when switching maps

The following crash was reported through Sentry:

Thread 28281 Crashed:
0   libtilededitor.so               0x7f1b4e333c47      Tiled::AbstractWorldTool::undoStack (abstractworldtool.cpp:397)
1   libtilededitor.so               0x7f1b4e4a5691      Tiled::MapEditor::updateActiveUndoStack (mapeditor.cpp:684)
2   libtilededitor.so               0x7f1b4e4acc34      Tiled::MapEditor::setCurrentDocument (mapeditor.cpp:379)
3   libtilededitor.so               0x7f1b4e3ed36c      Tiled::DocumentManager::currentIndexChanged (documentmanager.cpp:982)
4   libQt6Core.so.6                 0x7f1b4c797771      [inlined] QtPrivate::QSlotObjectBase::call (qobjectdefs_impl.h:363)
5   libQt6Core.so.6                 0x7f1b4c797771      doActivate<T> (qobject.cpp:3979)
6   libQt6Widgets.so.6              0x7f1b4dd6c5cd      QTabBar::currentChanged (moc_qtabbar.cpp:425)
7   libQt6Widgets.so.6              0x7f1b4dd73ad3      QTabBar::removeTab (qtabbar.cpp:1063)
8   libtilededitor.so               0x7f1b4e3f4228      Tiled::DocumentManager::closeDocumentAt (documentmanager.cpp:865)
9   libQt6Core.so.6                 0x7f1b4c797771      [inlined] QtPrivate::QSlotObjectBase::call (qobjectdefs_impl.h:363)
10  libQt6Core.so.6                 0x7f1b4c797771      doActivate<T> (qobject.cpp:3979)
11  libtilededitor.so               0x7f1b4e3ecaa4      Tiled::DocumentManager::documentCloseRequested (moc_documentmanager.cpp:635)
12  libQt6Core.so.6                 0x7f1b4c797771      [inlined] QtPrivate::QSlotObjectBase::call (qobjectdefs_impl.h:363)
13  libQt6Core.so.6                 0x7f1b4c797771      doActivate<T> (qobject.cpp:3979)
14  libQt6Widgets.so.6              0x7f1b4dd6c610      QTabBar::tabCloseRequested (moc_qtabbar.cpp:432)

The crash occurs on the last line of this function:

https://github.com/mapeditor/tiled/blob/cb4c33c599c811e0379cc4669e1bea77f258de3a/src/tiled/abstractworldtool.cpp#L392-L398

It is clear that even though WorldManager::worldForMap returned a World*, the DocumentManager::ensureWorldDocument did not have a matching WorldDocument instance. Matching instances are supposed to always be available based on these connections:

https://github.com/mapeditor/tiled/blob/cb4c33c599c811e0379cc4669e1bea77f258de3a/src/tiled/documentmanager.cpp#L278-L282

I'm not sure yet what could cause this issue, but I think this approach is a little too fragile in any case.

Start by tracing AbstractWorldTool::undoStack in src/tiled/abstractworldtool.cpp around lines 392-398, then follow MapEditor::updateActiveUndoStack, setCurrentDocument, and DocumentManager::closeDocumentAt from the stack trace. Compare this flow with the WorldDocument connections in src/tiled/documentmanager.cpp around lines 278-282. Done means reproducing the map-switch or document-close sequence and preventing the crash while preserving the expected undo-stack behavior.