How to access global variable $menu inside a class function

am puzzling why can't i access the global variable $menu inside a php class function

class adminMenus {
   public function getWPdefaultmenus(){
      global $menu;
      return $menu;
   }
}

using this code in a plugin file and this function still return NULL.

Edited what i am doing after @Will the Web Mechanic's answer

class adminMenus {
     private $getWPdefaultmenus;
     public function __construct(){
        add_action( 'admin_menu', array( $this, 'getWPdefaultmenus' ) );
      }
      public function getWPdefaultmenus(){
          global $menu;
          $this->getWPdefaultmenus = $menu;
      }
    }

but this returns error:Fatal error: Cannot access empty property