Whilst peer-reviewing another issue todo with the Atto editor (MDL-69292), I encountered another error when using the Atto editor. It only affects Moodle 4.0 onwards.

The issue is easy to reproduce. 

1. Log in to a Moodle 4.0+ instance as admin or teacher
2. Click on a course
3. Turn Edit mode on
4. Click on Add an activity or resource
5. Choose Page
6. Use the Atto editor (text editor)
7. Open up your browser's console
8. Click on the text/font dropdown in the editor (A symbol) and choose a style
9. You will see an error similar to TypeError: Cannot read properties of null. See attached screenshot.

This is not isolated to the Atto editor in the above instructions, it seems to be an error present on all Atto editors and is the same on all the major browsers (Chrome, Edge, Firefox and Safari).

This does not seems to impact performance and seems benign, but further investigation may reveal what is causing it and any negative impact.