john cook → credited br3dr3n → .
john cook → credited br3dr3n → .
john cook → credited br3dr3n → .
john cook → credited br3dr3n → .
john cook → credited br3dr3n → .
I searched the Drupal core (10.2) for the error message to look for similar code sections. (grep -rn "callbacks must be methods" .)
The are occurences in some DateTime classes.
lib/Drupal/Core/Datetime/Element/Datetime.php
lib/Drupal/Core/Datetime/Element/Datelist.php
There the callback is converted as follows:
Variable::callableToString($callback)
The Variable class is already in use in the Renderer class.