/**
  * @param WishListSetItem $item
  */
 public function addItem(WishListSetItem $item)
 {
     if (count($this->items) === 0) {
         $this->setItemAppId = $item->getAppId();
     } elseif ($this->setItemAppId !== $item->getAppId()) {
         throw new \InvalidArgumentException('you can not set different app ids for items in an item-set.');
     }
     $this->items[] = $item;
 }