In Jupyter Notebook, ipywidgets work fine, however they seem to not work in Jupyter Lab (which is supposedly better than Notebook).
I followed these directions.
Had the same issue, and what worked for me today was running the 'clean' command, as mentioned here: https://ipywidgets.readthedocs.io/en/latest/user_install.html#installing-the-jupyterlab-extension
So:
jupyter lab clean
jupyter labextension install @jupyter-widgets/jupyterlab-manager
And that got it working right for me just now.