Steps to reproduce the issue

I've recently built a J4 (4.1.5) website from scratch on a MAMP localhost running PHP 7.4.21 and everything has been working fine.

I updated the PHP version to 8.0.8 and attempted to created a streamlined client admin area having a Custom client admin (see https://magazine.joomla.org/all-issues/ ... experience) and everything worked fine UNTIL I tried to add a menu item to the client admin menu.

Expected result

When I select "Menu Item Type" a list of items should appear

Actual result

When I select "Menu Item Type" I receive the notification -

"An error has occurred.

0 strlen(): Argument #1 ($string) must be of type string, array given

System information (as much as possible)

Additional comments

If I rollback to PHP version 7.4.21 everything works as expected so I'm wondering if this might be a J4 / PHP 8 bug?