During shutdown, the gui attempts to update itself if a library closed plugin updates columns. This happens because the gui thinks the database is still open when the windows are being hidden.

src/calibre/gui2/ui.py

@@ -956,7 +956,7 @@ class Main(MainWindow, MainWindowMixin, DeviceMixin, EmailMixin,  # {{{
             mb.stop()
 
         if db is not None:
-            db.close()
+            self.library_view.model().close()
 
         try:
             try: