Fix threading bug (#780)

* Fix threading bug

* fix

nerfstudio/viewer/server/viewer_utils.py

@@ -711,6 +711,9 @@ class ViewerState:
         check_thread = CheckThread(state=self)
         render_thread = RenderThread(state=self, graph=graph, camera_ray_bundle=camera_ray_bundle)
 
+        check_thread.daemon = True
+        render_thread.daemon = True
+
         with TimeWriter(None, None, write=False) as vis_t:
             check_thread.start()
             render_thread.start()