"Error displaying widget" with jupyterlab on the littlest jupyterhub

On the littlest jupyterhub environment

The example works fine on jupyterhub but I have the message
"Error displaying widget" with jupyterlab

Jupyterlab 0.35.6
ipympl 0.3.3
Matplotlib 3.1.0
Jupyter-matplotlib 0.4.2

Start by reproducing the example in the littlest JupyterHub environment using the listed JupyterLab, ipympl, Matplotlib, and jupyter-matplotlib versions, then compare it with the working JupyterHub setup. Done means the example displays its widget in JupyterLab without the reported error.

Written by the indexing model from the issue text.