plugin XEP-0198: don't crash if transport is None in onAckTimeOut

sat/plugins/plugin_xep_0198.py

@@ -510,7 +510,11 @@ class XEP_0198(object):
     def onAckTimeOut(self, client):
         """Called when a requested ACK has not been received in time"""
         log.info(_(u"Ack was not received in time, aborting connection"))
-        client.xmlstream.transport.abortConnection()
+        transport = client.xmlstream.transport
+        if transport is None:
+            log.warning(u"transport was already removed")
+        else:
+            transport.abortConnection()
         client._xep_0198_session.req_timer = None