jupyter-webrtc doesn't play well with jupyterlab watch mode

[maartenbreddels]

I always get this issue with jupyter-webrtc (but I believe only the second time in run jupyter lab watch!)

$ jupyter labextension list                                                                                                                                                                                                       working
JupyterLab v0.33.0rc1
Known labextensions:
   app dir: /Users/maartenbreddels/miniconda3/envs/working/share/jupyter/lab
ipyvolume
        ipyvolume v0.5.2-dev.1  enabled  OK*
jupyter-matplotlib
        jupyter-matplotlib v0.3.0  enabled  OK
jupyter-threejs
        jupyter-threejs v1.0.0  enabled  OK
jupyter-webrtc
        jupyter-webrtc v0.4.1  enabled  OK*

   local extensions:
        ipyvolume: /Users/maartenbreddels/src/ipyvolume/js
        jupyter-webrtc: /Users/maartenbreddels/src/ipywebrtc/js

   linked packages:
        jupyter-webrtc: /Users/maartenbreddels/src/ipywebrtc/js

$ jupyter lab watch 

...........

> jupyter-webrtc@0.4.3 prepare /Users/maartenbreddels/src/ipywebrtc/js
> npm run build:lib


> jupyter-webrtc@0.4.3 build:lib /Users/maartenbreddels/src/ipywebrtc/js
> tsc --project .

jupyter-webrtc-0.4.3.tgz
Traceback (most recent call last):
  File "/Users/maartenbreddels/miniconda3/envs/working/bin/jupyter-lab", line 11, in <module>
    sys.exit(main())
  File "/Users/maartenbreddels/miniconda3/envs/working/lib/python3.6/site-packages/jupyter_core/application.py", line 266, in launch_instance
    return super(JupyterApp, cls).launch_instance(argv=argv, **kwargs)
  File "/Users/maartenbreddels/miniconda3/envs/working/lib/python3.6/site-packages/traitlets/config/application.py", line 657, in launch_instance
    app.initialize(argv)
  File "<decorator-gen-7>", line 2, in initialize
  File "/Users/maartenbreddels/miniconda3/envs/working/lib/python3.6/site-packages/traitlets/config/application.py", line 87, in catch_config_error
    return method(app, *args, **kwargs)
  File "/Users/maartenbreddels/miniconda3/envs/working/lib/python3.6/site-packages/notebook/notebookapp.py", line 1605, in initialize
    self.init_server_extensions()
  File "/Users/maartenbreddels/miniconda3/envs/working/lib/python3.6/site-packages/jupyterlab/labapp.py", line 213, in init_server_extensions
    load_jupyter_server_extension(self)
  File "/Users/maartenbreddels/miniconda3/envs/working/lib/python3.6/site-packages/jupyterlab/extension.py", line 145, in load_jupyter_server_extension
    watch(app_dir, logger)
  File "/Users/maartenbreddels/miniconda3/envs/working/lib/python3.6/site-packages/jupyterlab/commands.py", line 217, in watch
    return handler.watch()
  File "/Users/maartenbreddels/miniconda3/envs/working/lib/python3.6/site-packages/jupyterlab/commands.py", line 466, in watch
    self._populate_staging()
  File "/Users/maartenbreddels/miniconda3/envs/working/lib/python3.6/site-packages/jupyterlab/commands.py", line 920, in _populate_staging
    'linked_packages')
  File "/Users/maartenbreddels/miniconda3/envs/working/lib/python3.6/site-packages/jupyterlab/commands.py", line 1034, in _update_local
    os.remove(pjoin(dname, existing))
FileNotFoundError: [Errno 2] No such file or directory: '/Users/maartenbreddels/miniconda3/envs/working/share/jupyter/lab/staging/linked_packages/jupyter-webrtc-0.4.1-1146c25a4b8e99ba8bfe0daab26652cead4f710f.tgz'