Avoids errors being thrown by UI when designer is shutting down

* When shutting down, the dqp plugins change the Server Manager's state
  to SHUTDOWN. However, delete preview jobs may still be running and these
  change the server's configuration, which sparks notification events.

* The notifications events should not be distibuted around the system
  since everything is shutting down and little point in updating the UI

* More importantly it avoids calls to the Server's Manager's methods while
  not in a STARTED state.