[4.0] Cannot save template settings on frontend

[PhilETaylor]

Steps to reproduce the issue

install sample data
login to FRONTEND as admin
Edit template settings
http://joomla-cms/index.php/working-on-your-site/template-settings?view=templates

Click save

Expected result

Saved

Actual result

JSON response with error:

System information (as much as possible)

79fd949

Additional comments

[brianteeman]

Confirmed

[laoneo]

Please test #20620 which should fix the issue.