Don't forward events to uninitialized EventPlugin implementations

Another part of fixing #2090

src/octoprint/events.py

@@ -163,7 +163,8 @@ class EventManager(object):
 
 				octoprint.plugin.call_plugin(octoprint.plugin.types.EventHandlerPlugin,
 				                             "on_event",
-				                             args=(event, payload))
+				                             args=(event, payload),
+				                             initialized=True)
 			self._logger.info("Event loop shut down")
 		except:
 			self._logger.exception("Ooops, the event bus worker loop crashed")