Steps to reproduce the issue

Go to System / Site Templates / Cassiopeia Details and Files
Create a New File / user.css in the css folder

Expected result

Creation works OK but...

Actual result

This message appears above the Edit box:

Warning: Undefined property: stdClass::$readOnly in /Users/ceford/Sites/j4cms/plugins/editors/codemirror/layouts/editors/codemirror/element.php on line 41

System information (as much as possible)

Cloned joomla-cms from repo, built with npm ci, installed
PHP 8.1.9

Additional comments

Debug set to Yes and Error Reporting set to Maximum