"Error displaying widget" using Jupyter Lab 1.0.2

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Possibly related to #124, I am trying to run a super simple example after following the installation instructions:

```
pip install ipympl

# If using JupyterLab
# Install nodejs: https://nodejs.org/en/download/
jupyter labextension install @jupyter-widgets/jupyterlab-manager
jupyter labextension install jupyter-matplotlib
```

nodejs info:
```
$ node -v
v10.16.0
$ npm version
{ npm: '6.9.0',
...
}
```

Jupyter info:
```
$ jupyter --version
jupyter core : 4.5.0
jupyter-notebook : 6.0.0
qtconsole : not installed
ipython : 7.6.1
ipykernel : 5.1.1
jupyter client : 5.3.1
jupyter lab : 1.0.2
nbconvert : 5.5.0
ipywidgets : 7.5.0
nbformat : 4.4.0
traitlets : 4.3.2
```

Simple example:
```
# Enabling the `widget` backend.
# This requires jupyter-matplotlib a.k.a. ipympl.
# ipympl can be install via pip or conda.
%matplotlib widget
# aka import ipympl
 
import matplotlib.pyplot as plt
 
plt.plot([0, 1, 2, 2])
plt.show()
```

Error:
```
Error displaying widget
```

Not sure what else I need, or where a version mismatch is, any help is appreciated. Thank you!

Start by reproducing the example with the listed ipympl, JupyterLab, Node.js, and Jupyter package versions and the shown labextension commands. Check the JupyterLab widget-extension setup and version compatibility; done means the example displays the plotted widget instead of "Error displaying widget".

Written by the indexing model from the issue text.