xeus-python: model.get(list_trait) return null if the list_trait is empty

hi,

Please see very simple example

from ipywidgets import *
Text("hello")

In the JS console. I see error:

utils.js:35 Uncaught (in promise) TypeError: Cannot read property 'filter' of null
    at h (utils.js:35)
    at e.update_classes (widget.js:801)
    at widget.js:731

line 731: _this.update_classes([], _this.model.get('_dom_classes')); In IPython, I think _this.model.get('_dom_classes') returns an empty list in this case.