/**
  * An ajax callback function that creates a new custom field with default settings.
  **/
 public function create_custom_field_ajax()
 {
     $label = $_POST['label'];
     $meta_key = $_POST['meta_key'];
     $defaults = $this->form_manager->get_defaults();
     if (!($label && $meta_key)) {
         $response = array('success' => false, 'error' => __('You missed a required item.', 'wpfepp-plugin'));
         die(json_encode($response));
     }
     if ($meta_key != sanitize_key($meta_key)) {
         $response = array('success' => false, 'error' => __('The meta key is invalid.', 'wpfepp-plugin'));
         die(json_encode($response));
     }
     $field_settings = array_merge(array('widget_label' => $label, 'label' => $label), $defaults['custom_field']);
     ob_start();
     $this->display_item($meta_key, $field_settings);
     $widget_html = ob_get_clean();
     $response = array('success' => true, 'widget_html' => $widget_html);
     die(json_encode($response));
 }