Possible bug in our ContentErrorBoundary

[peterbe]

Summary When exploring https://github.com/mdn/kuma/issues/6725 I noticed a second error within the error boundary stuff. In particular, the proptype warning:

Warning: Failed prop type: The prop `children` is marked as required in `ErrorMessage`, but its value is `undefined`.

Steps To Reproduce (STR)

1. Do something that causes a crash within article.jsx in client-side. E.g. https://github.com/mdn/kuma/issues/6725
2. View the page (on the read-only domain)
3. Observer the proptype warning about ContentErrorBoundary

Actual behavior Warning.

Expected behavior No propType warning.

Additional context Might be harmless. Who knows. Still noise.