Notebook rendered with Voila does not show Axes in a matplotlib widget

## Describe the issue
When rendering a notebook using Voila, the interactive matplotlib widget using ipympl backend is also rendered. However this widget is completely missing the Axes, therefore not showing any data at all.

A minimum reproducible code would be
```python
%matplotlib widget

plt.ioff()
figRHF = plt.figure(figsize=(10, 5))
figRHF.canvas.layout.min_height = '400px'
axRHF = figRHF.gca()
test = axRHF.plot([0, 1], [0, 1])
print(axRHF)
print(figRHF)
print(figRHF.gca())
print(test)

display(
figRHF.canvas
)
```
And this is the output from this code, on the left is the Jupyter Lab notebook output, on the right is the Voila output

## Versions

```
'3.8.12 | packaged by conda-forge | (default, Oct 12 2021, 21:22:46) [MSC v.1916 64 bit (AMD64)]'
ipympl version: 0.8.2

Selected Jupyter core packages...
IPython : 7.29.0
ipykernel : 6.5.0
ipywidgets : 7.6.5
jupyter_client : 6.1.12
jupyter_core : 4.9.1
jupyter_server : 1.11.2
jupyterlab : 3.2.4
nbclient : 0.5.9
nbconvert : 6.3.0
nbformat : 5.1.3
notebook : 6.4.6
qtconsole : not installed
traitlets : 5.1.1

Known nbextensions:
config dir: C:\Users\are014\Miniconda3\envs\humaninjurymodel\etc\jupyter\nbconfig
notebook section
ipydatetime/extension enabled
jupyter-matplotlib/extension enabled
voila/extension enabled
jupyter-js-widgets/extension enabled

- Validating: ok
- Validating: ok
- Validating: ok
- Validating: ok

JupyterLab v3.2.4
C:\Users\are014\Miniconda3\envs\humaninjurymodel\share\jupyter\labextensions
jupyter-matplotlib v0.10.2 enabled ok
@jupyter-widgets/jupyterlab-manager v3.0.1 enabled ok (python, jupyterlab_widgets)
@voila-dashboards/jupyterlab-preview v2.1.0 enabled ok (python, voila)

Other labextensions (built into JupyterLab)
app dir: C:\Users\are014\Miniconda3\envs\humaninjurymodel\share\jupyter\lab
jupyter-widget-datetime v1.2.0 enabled ok

```

Start by running the minimal reproducible example with the listed ipympl, JupyterLab, ipywidgets, and Voila versions, then compare the widget output in JupyterLab and Voila. Done means the Voila-rendered matplotlib widget displays its Axes and plotted data as it does in JupyterLab.

Written by the indexing model from the issue text.