hack around premature completer deletion

haven't been able to identify why qt sometimes deletes the completer
early, so just suppress the error for now

https://anki.tenderapp.com/discussions/beta-testing/884-freeze-and-sigabrt-when-add-dialog-is-left-open-for-about-one-hour-and-used-afterwards#comment_44837119

aqt/tagedit.py

@@ -79,6 +79,8 @@ class TagEdit(QLineEdit):
         self.completer.popup().hide()
 
     def hideCompleter(self):
+        if sip.isdeleted(self.completer):
+            return
         self.completer.popup().hide()
 
 class TagCompleter(QCompleter):