This site is running a Roots child theme. The Roots code is from 2014; not sure whether it would still be a problem with the current build. I’ve been using Piklist with Roots for a couple of years; this error just started showing up in the last week or two. (I’m guessing it first appeared with 0.9.9.)

The error messages show up only on the admin dashboard page, and are 16 repetitions of the following:

Warning: in_array() expects parameter 2 to be array, null given in 
/Users/donna/Sites/marys/wp-content/plugins/piklist/includes/class-piklist-meta.php 
on line 340

I finally narrowed it down to the following code in the Roots theme. If I comment out the admin_init action, the error messages go away.

/**
 * Remove unnecessary dashboard widgets
 *
 * @link http://www.deluxeblogtips.com/2011/01/remove-dashboard-widgets-in-wordpress.html
 */
function roots_remove_dashboard_widgets() {
  remove_meta_box('dashboard_incoming_links', 'dashboard', 'normal');
  remove_meta_box('dashboard_plugins', 'dashboard', 'normal');
  remove_meta_box('dashboard_primary', 'dashboard', 'normal');
  remove_meta_box('dashboard_secondary', 'dashboard', 'normal');
}
add_action('admin_init', 'roots_remove_dashboard_widgets');

HTH,
Donna