Avoid exception on exit

Closing anki when the preferences dialog
is open will cause an exception since
self.mw.col in Preferences is not available
anymore.

Avoid the exception by checking self.mw.col
again on accept().

aqt/preferences.py

@@ -29,6 +29,9 @@ class Preferences(QDialog):
         self.show()
 
     def accept(self):
+        # avoid exception if main window is already closed
+        if not self.mw.col:
+            return
         self.updateCollection()
         self.updateNetwork()
         self.updateBackup()