/websocket returns 500 internal server error

[y-kim]

It occurs from the latest docker image.

Logs from the docker instance:

Traceback (most recent call last):

  File "/usr/local/lib/python3.6/site-packages/bottle.py", line 868, in _handle
    return route.call(**args)

  File "/usr/local/lib/python3.6/site-packages/bottle.py", line 1748, in wrapper
    rv = callback(*a, **ka)

  File "/usr/src/app/bottle_websocket/plugin.py", line 5, in wrapper
    callback(request.environ.get('wsgi.websocket'), *args, **kwargs)

  File "/usr/src/app/youtube-dl-server.py", line 59, in echo
    msg = WSAddr.wsClassVal.receive()

  AttributeError: 'NoneType' object has no attribute 'receive'