I was getting this a fatal error on a wp site…

Fatal error: Call to undefined function get_current_screen() in /home/slv01/www/public_html/wp-content/plugins/wp-optimize/includes/class-updraft-smush-manager.php on line 1048

The solution? Add

require_once(ABSPATH . 'wp-admin/includes/screen.php');

to your theme/function.php