WebSocket error 502 on mybinder.org about jupyterlab-demo HOT 2 CLOSED

[E 21:38:48.121 NotebookApp] Uncaught exception GET /user/jupyterlab-jupyterlab-demo-sd115y5w/notebooks/demo/Lorenz.ipynb (162.238.29.51)
    HTTPServerRequest(protocol='https', host='hub.mybinder.org', method='GET', uri='/user/jupyterlab-jupyterlab-demo-sd115y5w/notebooks/demo/Lorenz.ipynb', version='HTTP/1.1', remote_ip='162.238.29.51')
    Traceback (most recent call last):
      File "/srv/conda/lib/python3.6/site-packages/tornado/web.py", line 1697, in _execute
        result = method(*self.path_args, **self.path_kwargs)
      File "/srv/conda/lib/python3.6/site-packages/tornado/web.py", line 3174, in wrapper
        return method(self, *args, **kwargs)
      File "/srv/conda/lib/python3.6/site-packages/notebook/notebook/handlers.py", line 59, in get
        get_custom_frontend_exporters=get_custom_frontend_exporters
      File "/srv/conda/lib/python3.6/site-packages/notebook/base/handlers.py", line 463, in render_template
        return template.render(**ns)
      File "/srv/conda/lib/python3.6/site-packages/jinja2/asyncsupport.py", line 76, in render
        return original_render(self, *args, **kwargs)
      File "/srv/conda/lib/python3.6/site-packages/jinja2/environment.py", line 1008, in render
        return self.environment.handle_exception(exc_info, True)
      File "/srv/conda/lib/python3.6/site-packages/jinja2/environment.py", line 780, in handle_exception
        reraise(exc_type, exc_value, tb)
      File "/srv/conda/lib/python3.6/site-packages/jinja2/_compat.py", line 37, in reraise
        raise value.with_traceback(tb)
      File "/srv/conda/lib/python3.6/site-packages/notebook/templates/notebook.html", line 1, in top-level template code
        {% extends "page.html" %}
      File "/etc/jupyter/templates/page.html", line 1, in top-level template code
        {% extends "templates/page.html" %}
      File "/srv/conda/lib/python3.6/site-packages/notebook/templates/page.html", line 154, in top-level template code
        {% block header %}
      File "/srv/conda/lib/python3.6/site-packages/notebook/templates/notebook.html", line 120, in block "header"
        {% for exporter in get_custom_frontend_exporters() %}
      File "/srv/conda/lib/python3.6/site-packages/notebook/notebook/handlers.py", line 19, in get_custom_frontend_exporters
        from nbconvert.exporters.base import get_export_names, get_exporter
      File "/srv/conda/lib/python3.6/site-packages/nbconvert/__init__.py", line 7, in <module>
        from . import postprocessors
      File "/srv/conda/lib/python3.6/site-packages/nbconvert/postprocessors/__init__.py", line 5, in <module>
        from .serve import ServePostProcessor
      File "/srv/conda/lib/python3.6/site-packages/nbconvert/postprocessors/serve.py", line 19, in <module>
        class ProxyHandler(web.RequestHandler):
      File "/srv/conda/lib/python3.6/site-packages/nbconvert/postprocessors/serve.py", line 21, in ProxyHandler
        @web.asynchronous
    AttributeError: module 'tornado.web' has no attribute 'asynchronous'