matplotlib widget occasionally fails to render.

### Bug report

**Bug summary**

Matplotlib widget occasionally fails to render.

**Code for reproduction**

```python
%matplotlib widget
from matplotlib import pyplot
fig, axes = pyplot.subplots(1, 2, figsize=(8, 4))
pyplot.show()
raise
```

**Actual outcome**


**Expected outcome**

Two empty subplots.

**Matplotlib version**

* Operating system: some sort of linux running docker running jupyterlab
* Matplotlib version: 3.2.1
* Matplotlib backend (`print(matplotlib.get_backend())`): module://ipympl.backend_nbagg
* Python version: 3.8.2
* Jupyter version (if applicable): jupyterlab 2.0.2
* Other libraries:

Start by running the provided reproduction with the ipympl backend in JupyterLab 2.0.2 and the reported environment. Investigate why the widget intermittently fails to render, using the expected result of two empty subplots as the completion criterion. No source file or test is identified in the issue, so locating the relevant rendering code and adding coverage will require project research.

Written by the indexing model from the issue text.