Fix double-free corruption on exit

The mDecorations QTreeWidgetItem is owned by the QTreeWidget, which is owned
by KOPrefsDialogPlugins' parent widget. So when the parent widget deletes
the QTreeWidget, the decorations tree item is deleted as well and once
we enter KOPrefsDialogPlugins' destructor, mDecorations is a dangling
pointer by then.

(cherry picked from commit 87c20da4)

BUG: 483336
FIXED-IN: 24.02.2