Only "Loading widget..." but no Matplotlib figures (fresh install with conda)

Fresh install with conda (laptop with Ubuntu 18.04 => nothing to do with Docker).

Maybe related to #161 ?

Matplotlib figures do not show up and I get only

```
Loading widget...
[]
```

Standard installation with conda:
```
conda install -c conda-forge ipympl nodejs
jupyter labextension install @jupyter-widgets/jupyterlab-manager jupyter-matplotlib
```

```
jupyter labextension list
JupyterLab v1.2.5
Known labextensions:
app dir: /home/marianne/miniconda3/share/jupyter/lab
@jupyter-widgets/jupyterlab-manager v1.1.0 enabled OK
jupyter-matplotlib v0.5.0 enabled OK
```

Note that ipywidgets seems to work fine. `from ipywidgets import Button; Button(description='test')` gives a button.

Reproduce the issue using the listed conda installation and JupyterLab 1.2.5 setup, then compare the working ipywidgets Button with the matplotlib output that remains on “Loading widget...”. The fix is complete when matplotlib figures render instead of only showing the loading message.

Written by the indexing model from the issue text.