I actually get the error again on another website. I have the latest Piklist and still get the error: Notice: Trying to access array offset on value of type bool in /var/www/moumou.dk/cc/wp-content/plugins/piklist/includes/class-piklist-meta.php on line 126
If I change line 126 from if(!is_null($meta_box['id']))
to if($meta_box && !is_null($meta_box['id']))
It seems to work and throws no errors.