src/Pure/PIDE/session.scala

               if (prover.defined) protocol_handlers.exit()
               for (id <- global_state.value.theories.keys) {
                 val snapshot = global_state.change_result(_.end_theory(id))
                 finished_theories.post(snapshot)
               }
-              file_formats.stop_session
+              file_formats.stop_session()
               phase = Session.Terminated(result)
               prover.reset()
 
             case _ =>
               raw_output_messages.post(output)