widgets error (websocket): in callback `on-message' Publisher's subscribers had errors

[gmoutso]

I want to try out the experimental widgets support. My hope is that I will be able to use python pyviz (namely panel with ipywidgets) in a docker/ssh environment where only one port is accessible (the jupyter server's). I am not sure if what I want would work, so I tried something simpler. I run a repl connected locally to a jupyter server. Inside I run

import ipywidgets
slider = ipywidgets.IntSlider(start=0, end=10)
slider

Is this supposed to work? I get an IntSlider in the output of the REPL, but no browser pop-up and this warning pops up instead

Error (websocket): in callback `on-message': Publisher's subscribers had errors: (jupyter-publisher-subscribers-had-errors (json-error ....

This error seems to be different to #491 that says invalid-slot-name.