instance check: bux fix in shutdown() - repairs falling at assert when reiniting otherInstanceMessageHandler

src/slic3r/GUI/InstanceCheck.cpp

@@ -298,6 +298,7 @@ void OtherInstanceMessageHandler::shutdown(MainFrame* main_frame)
 			m_stop = false;
 		}
 #endif //BACKGROUND_MESSAGE_LISTENER
+		m_callback_evt_handler = nullptr;
 		m_initialized = false;
 	}
 }