connection: Fix deregisterevent tracebacks on app close

virtManager/connection.py

@@ -924,7 +924,7 @@ class vmmConnection(vmmGObject):
             for dev in devs.values():
                 dev.cleanup()
 
-        if self._backend:
+        if not self._backend.is_closed():
             if self._domain_cb_id is None:
                 self._backend.domainEventDeregisterAny(self._domain_cb_id)
             self._domain_cb_id = None

virtinst/connection.py

@@ -137,6 +137,9 @@ class VirtualConnection(object):
     # Public API #
     ##############
 
+    def is_closed(self):
+        return not bool(self._libvirtconn)
+
     def close(self):
         self._libvirtconn = None
         self._uri = None