qt_mainwindow_layout(mainWindow) returns the pointer to a stale object,
if the layout has already been deleted.
Most users of qt_mainwindow_layout() assert the returned pointer, or
handle nullptr explicitly. If the mainwindow layout has already been
deleted, they will still access it. While this has not lead to (known)
issues yet, a UAF exception is thrown in an ASAN build.
=> Use a QPointer instead of a bare pointer.
=> Add missing nullptr check in QDockWidgetPrivate::endDrag()
Pick-to: 6.6 6.5
Change-Id: I3484b53e0e2213b82085ceed3ffa66913947189f
Reviewed-by: David Faure <david.faure@kdab.com>
1914f62c31