Possible problem: you are starting session while already having some data in output buffer*

Caute, neviete mi poradit? Mam komponentu ShoppingCartBlock ktora si vola service ShoppingCart pri nacitani stranky, problem je v tom ze ked v render zavolam service ShoppingCart ktora by mala vyrobit session pokial este neexistuje,tak mi vypise error:

Possible problem: you are starting session while already having some data in output buffer. This may not work if the outputted data grows. Try starting the session earlier.

pokial urobim dalsi refresh tak uz to funguje, neviete mi poradit ako toto riesit?

komponenta: ShoppingCartBlock

class ShoppingCartBlockControl extends BaseControl
{
	private $shoppingCart;

	/**
	 * @param ShoppingCart $shoppingCart
	 */
	public function __construct(ShoppingCart $shoppingCart)
	{
		parent::__construct();
		$this->shoppingCart = $shoppingCart;
	}

	public function render()
	{
		$shoppingCart = $this->shoppingCart->getItems();
		$this->template->shoppingCart = $shoppingCart ;
		$this->template->render();
	}

}

a service:

class ShoppingCart extends Nette\Object {

    /** @var Nette\Http\SessionSection */
    private $cart;

    public function __construct(Nette\Http\Session $session) {
        $this->cart = $session->getSection(__CLASS__);
    }

    public function getItems() {
        return $this->cart->getIterator()->getArrayCopy();
    }

}