public static function get_instance()
 {
     // If the single instance hasn't been set, set it now.
     if (null == self::$instance) {
         self::$instance = new self();
     }
     return self::$instance;
 }
 public function add_plugin_meta_boxes()
 {
     $includes = array('class-meta-box-tables');
     foreach ($includes as $file) {
         include_once 'includes/' . $file . '.php';
     }
     // Reviews Meta Box
     $reviews_meta_box = RWP_Reviews_Meta_Box::get_instance();
     $reviews_meta_box->init();
     // Tables Meta Box
     $tables_meta_box = RWP_Tables_Meta_Box::get_instance();
     $tables_meta_box->init();
 }