WP Shopify breaks Ninja Forms in the admin

Your plugin has a bug in it that is removing admin body classes added by other plugins. In classes/class-backend.php the wps_admin_body_class method returns nothing in various places. You have to return $classes and then append your classes to it.

This is the fixed code:

   public function wps_admin_body_class( $classes ) {
      
      // If the settings aren't empty ...
      if (empty($this->plugin_settings['general'])) {
         return $classes;
      }

      // If the right admin page ...
      $screen_id = $this->get_screen_id();

      if ($screen_id !== 'edit-wps_products' && $screen_id !== 'edit-wps_collections') {
         return $classes;
      }

      if (!$this->plugin_settings['general']->is_syncing_posts) {
         return  $classes . ' wps-is-lite-sync';
      }
      
      return $classes;

   }

Please fix your plugin.