Warning call_user_func_array() expects parameter 1 to be a valid callback

Warning: call_user_func_array() expects parameter 1 to be a valid callback. This is a recurring error when dealing with actions/filter in WordPress. In my case, it is usually caused by a filter or an action not properly declared. ... add_action ( 'action_tag' , array( $this , 'my_callback , 30 ) );