embed_minimal_html fails

[kerchner]

With Python 3.6.7 and having installed jupyter labextension install @jupyter-widgets/jupyterlab-manager@0.38, using embed_minimal_html() results in the following error:

1 from ipywidgets.embed import embed_minimal_html
      2 
----> 3 embed_minimal_html('carpentries_instructors_basic.html', views=[map], title='The Carpentries Instructors')

~/anaconda3/envs/jupyter-maps/lib/python3.6/site-packages/ipywidgets/embed.py in embed_minimal_html(fp, views, title, template, **kwargs)
    300     {embed_kwargs}
    301     """
--> 302     snippet = embed_snippet(views, **kwargs)
    303 
    304     values = {

~/anaconda3/envs/jupyter-maps/lib/python3.6/site-packages/ipywidgets/embed.py in embed_snippet(views, drop_defaults, state, indent, embed_url, requirejs, cors)
    277     values = {
    278         'load': load.format(embed_url=embed_url, use_cors=use_cors),
--> 279         'json_data': escape_script(json.dumps(data['manager_state'], indent=indent)),
    280         'widget_views': widget_views,
    281     }

~/anaconda3/envs/jupyter-maps/lib/python3.6/json/__init__.py in dumps(obj, skipkeys, ensure_ascii, check_circular, allow_nan, cls, indent, separators, default, sort_keys, **kw)
    236         check_circular=check_circular, allow_nan=allow_nan, indent=indent,
    237         separators=separators, default=default, sort_keys=sort_keys,
--> 238         **kw).encode(obj)
    239 
    240 

~/anaconda3/envs/jupyter-maps/lib/python3.6/json/encoder.py in encode(self, o)
    199         chunks = self.iterencode(o, _one_shot=True)
    200         if not isinstance(chunks, (list, tuple)):
--> 201             chunks = list(chunks)
    202         return ''.join(chunks)
    203 

~/anaconda3/envs/jupyter-maps/lib/python3.6/json/encoder.py in _iterencode(o, _current_indent_level)
    428             yield from _iterencode_list(o, _current_indent_level)
    429         elif isinstance(o, dict):
--> 430             yield from _iterencode_dict(o, _current_indent_level)
    431         else:
    432             if markers is not None:

~/anaconda3/envs/jupyter-maps/lib/python3.6/json/encoder.py in _iterencode_dict(dct, _current_indent_level)
    402                 else:
    403                     chunks = _iterencode(value, _current_indent_level)
--> 404                 yield from chunks
    405         if newline_indent is not None:
    406             _current_indent_level -= 1

~/anaconda3/envs/jupyter-maps/lib/python3.6/json/encoder.py in _iterencode_dict(dct, _current_indent_level)
    402                 else:
    403                     chunks = _iterencode(value, _current_indent_level)
--> 404                 yield from chunks
    405         if newline_indent is not None:
    406             _current_indent_level -= 1

~/anaconda3/envs/jupyter-maps/lib/python3.6/json/encoder.py in _iterencode_dict(dct, _current_indent_level)
    402                 else:
    403                     chunks = _iterencode(value, _current_indent_level)
--> 404                 yield from chunks
    405         if newline_indent is not None:
    406             _current_indent_level -= 1

~/anaconda3/envs/jupyter-maps/lib/python3.6/json/encoder.py in _iterencode_dict(dct, _current_indent_level)
    402                 else:
    403                     chunks = _iterencode(value, _current_indent_level)
--> 404                 yield from chunks
    405         if newline_indent is not None:
    406             _current_indent_level -= 1

~/anaconda3/envs/jupyter-maps/lib/python3.6/json/encoder.py in _iterencode_list(lst, _current_indent_level)
    323                 else:
    324                     chunks = _iterencode(value, _current_indent_level)
--> 325                 yield from chunks
    326         if newline_indent is not None:
    327             _current_indent_level -= 1

~/anaconda3/envs/jupyter-maps/lib/python3.6/json/encoder.py in _iterencode(o, _current_indent_level)
    435                     raise ValueError("Circular reference detected")
    436                 markers[markerid] = o
--> 437             o = _default(o)
    438             yield from _iterencode(o, _current_indent_level)
    439             if markers is not None:

~/anaconda3/envs/jupyter-maps/lib/python3.6/json/encoder.py in default(self, o)
    178         """
    179         raise TypeError("Object of type '%s' is not JSON serializable" %
--> 180                         o.__class__.__name__)
    181 
    182     def encode(self, o):

TypeError: Object of type 'MetaHasTraits' is not JSON serializable