Error message in admin

Thank you for reporting this, I can see there is a minor bug causing the error. Can I ask you what version of PHP you are using?

The error should not cause any harm, but if you want to remove the message right now, go to the mentioned file, line 99 and replace:

add_action('add_meta_boxes',array(__CLASS__,'add_group_meta_box'),10,3);

with:

add_action('add_meta_boxes',array(__CLASS__,'add_group_meta_box'),10,2);