EDMarketConnector: Bail in AppWindow.journal_entry if monitor.thread is None

monitor.thread should only be none when there's a <<JournalEvent>> to
process if we're in shutdown, in which case we do *not* want to be
processing journal events.

EDMarketConnector.py

@@ -743,6 +743,10 @@ class AppWindow(object):
                 'FlightCon':  _('Helm'),  # Multicrew role
             }.get(role, role)
 
+        if monitor.thread is None:
+            logger.debug('monitor.thread is None, assuming shutdown and returning')
+            return
+
         while True:
             entry = monitor.get_entry()
             if not entry: