Meta boxes not showing up since WP 4.4 update

After comparing two copies of a site using this plugin, you can see it breaks with the WP update to 4.4

The following notice was produced when WP_DEBUG is turned on:

Notice: convert_to_screen(), add_meta_box() was called incorrectly. Likely direct inclusion of wp-admin/includes/template.php in order to use add_meta_box(). This is very wrong. Hook the add_meta_box() call into the add_meta_boxes action instead.

https://wordpress.org/plugins/custom-field-template/