PHP 8.1
While testing a component of mine against PHP 8.1 and J4, I got this Deprecation Notices that seemed relevant even outside the scope of my own devt:

Deprecated: ucfirst(): Passing null to parameter #1 ($string) of type string is deprecated in /libraries/src/MVC/Controller/BaseController.php on line 318

Deprecated: strtolower(): Passing null to parameter #1 ($string) of type string is deprecated in /libraries/src/MVC/Controller/BaseController.php on line 713

My suggestions:

line 318
$class = ucfirst((string) $prefix) . 'Controller' . ucfirst((string) $type);

line 712
$task = strtolower((string) $task);