title: unable to download nbextensions' javascript and css file
body: <p>I am trying to install nbextensions to my jupyter notebook. When opening the notebook, I can see the tab for the nbextensions but is is empty. There is a reload button moving but nothing ever shows.
  I went to command prompt and the following commands were executed successfully:</p>
  
  <pre><code>pip install jupyter_contrib_nbextensions
  pip install https://github.com/ipython-contrib/jupyter_contrib_nbextensions/tarball/master
  conda install -c conda-forge jupyter_contrib_nbextensions
  </code></pre>
  
  <p>But when I try to execute</p>
  
  <pre><code>jupyter contrib nbextension install --user
  </code></pre>
  
  <p>it seems like all extensions validate but when writing them there is a multitude of errors.</p>
  
  <pre><code>[I 09:49:34 InstallContribNbextensionsApp] - Validating: ok
  [I 09:49:34 InstallContribNbextensionsApp] Installing jupyter_contrib_nbextensions items to config in C:\Users\sammy\.jupyter
  Enabling: jupyter_nbextensions_configurator
  - Writing config: C:\Users\sammy\.jupyter
      - Validating...
        jupyter_nbextensions_configurator  ok
  Enabling notebook nbextension nbextensions_configurator/config_menu/main...
  Traceback (most recent call last):
    File "C:\Users\sammy\Anaconda3\Scripts\jupyter-contrib-script.py", line 5, in &lt;module&gt;
      sys.exit(jupyter_contrib_core.application.main())
    File "C:\Users\sammy\Anaconda3\lib\site-packages\jupyter_core\application.py", line 266, in launch_instance
      return super(JupyterApp, cls).launch_instance(argv=argv, **kwargs)
    File "C:\Users\sammy\Anaconda3\lib\site-packages\traitlets\config\application.py", line 658, in launch_instance
      app.start()
    File "C:\Users\sammy\Anaconda3\lib\site-packages\jupyter_contrib_core\application.py", line 50, in start
      super(JupyterContribApp, self).start()
    File "C:\Users\sammy\Anaconda3\lib\site-packages\jupyter_core\application.py", line 255, in start
      self.subapp.start()
    File "C:\Users\sammy\Anaconda3\lib\site-packages\jupyter_contrib_nbextensions\application.py", line 226, in start
      super(ContribNbextensionsApp, self).start()
    File "C:\Users\sammy\Anaconda3\lib\site-packages\jupyter_core\application.py", line 255, in start
      self.subapp.start()
    File "C:\Users\sammy\Anaconda3\lib\site-packages\jupyter_contrib_nbextensions\application.py", line 175, in start
      toggle_install_config(self._toggle_value, **kwargs)
    File "C:\Users\sammy\Anaconda3\lib\site-packages\jupyter_contrib_nbextensions\install.py", line 103, in toggle_install_config
      configurator_app.start()
    File "C:\Users\sammy\Anaconda3\lib\site-packages\jupyter_nbextensions_configurator\application.py", line 77, in start
      user=self.user, sys_prefix=self.sys_prefix)
    File "C:\Users\sammy\Anaconda3\lib\site-packages\notebook\nbextensions.py", line 351, in _set_nbextension_state
      cm.update(section, {"load_extensions": {require: state}})
    File "C:\Users\sammy\Anaconda3\lib\site-packages\traitlets\config\manager.py", line 85, in update
      data = self.get(section_name)
    File "C:\Users\sammy\Anaconda3\lib\site-packages\traitlets\config\manager.py", line 63, in get
      return json.load(f)
    File "C:\Users\sammy\Anaconda3\lib\json\__init__.py", line 299, in load
      parse_constant=parse_constant, object_pairs_hook=object_pairs_hook, **kw)
    File "C:\Users\sammy\Anaconda3\lib\json\__init__.py", line 354, in loads
      return _default_decoder.decode(s)
    File "C:\Users\sammy\Anaconda3\lib\json\decoder.py", line 339, in decode
      obj, end = self.raw_decode(s, idx=_w(s, 0).end())
    File "C:\Users\sammy\Anaconda3\lib\json\decoder.py", line 357, in raw_decode
      raise JSONDecodeError("Expecting value", s, err.value) from None
  json.decoder.JSONDecodeError: Expecting value: line 6 column 22 (char 137)
  </code></pre>
  
  <p>Any idea where the issue could be? Many thanks!</p>