Bug report

Extracting the extra bug discovered in #19116 (comment)

Bug summary
The code in https://matplotlib.org/devdocs/gallery/user_interfaces/embedding_webagg_sgskip.html does not display anything in the browser when running master matplotlib (27aaf0c)

Code for reproduction

• Download the example from the docs
• run it using latest matplotlib
• open http://127.0.0.1:8080/
• See no plot

Actual outcome

No plot + this DOM exception in the JS console:

Uncaught DOMException: An attempt was made to use an object that is not, or is no longer, usable mpl.js:394
    send_message http://127.0.0.1:8080/mpl.js:394
    _init_canvas http://127.0.0.1:8080/mpl.js:161
    figure http://127.0.0.1:8080/mpl.js:55
    <anonymous> http://127.0.0.1:8080/:40

Expected outcome
A working interactive plot shows up

Matplotlib version

• Operating system: Ubuntu 20.04
• Matplotlib version: 27aaf0c
• Matplotlib backend (print(matplotlib.get_backend())): webAgg
• Python version: 3.9.0
• Browser: Firefox 83.0

Installed python from conda-forge:

Python 3.9.0 | packaged by conda-forge | (default, Oct 14 2020, 22:59:50) 
[GCC 7.5.0] on linux