The error exists only in the Frontend, not in the Admin-Tool, if you click on the Editor-Buttons of Modulesanywhere, Articlesanywhere, contenttemplater. (maybe more, i've only installed this three).
If you change the method from "Open modal:yes" to "no" (show dropdown) there is no error anymore.
The error showed up after upgrading to PHP 8.0 and didn't exist in PHP 7.4
J3 latest version
T4 frame latest version
PHP 8.0
All regular labs plugins latest version
Going by the error, this is an error concerning your template.
I don't think so. The error appears only in the Frontend, not in the Admin-Tool. So there must be a diffence in the admin- and frontend-code. And only when you use the Regular-Labs Modal for Choosing.
Going by your screenshot, this concerns Content Templater.
When you set Content Templater to open the list of templates in a modal, it is NOT using the Regular Labs Modals extension.
It is using Joomla's built-in modal functionality, which is used by different editor buttons.
Modules Anywhere, Articles Anywhere and others will always use that modal functionality.
It will then load a frontend page in the modal to show what it should. Frontend views go through your template.
And again, that error suggests it has to do with your frontend template.
You can test this by switching your frontend template to the Joomla core template protostar.
Then try to edit an article in the frontend and click on those editor buttons.