Confirmed this offline with Brian Shaughnessy and also on the latest civicrm Joomla 4.3 demo site.
Steps to reproduce:
1. In CiviCRM Admin backend set Administrator: Settings - Display Preferences set WYSIWYG Editor to Joomla Default editor
TypeError: Type error
Seems that if mootools is loaded in Joomla backend such an error occurs.
This was not the case with civiCRM 4.2.7.