Ajax Cart updated only after user go to another page or refresh page

-- url of the page with the problem -- : www.montinibags.it
-- HikaShop version -- : 3.4.0
-- Joomla version -- : 3.8.8
-- PHP version -- : 5.6.33
-- Browser(s) name and version -- : chrome any version
-- Error-message(debug-mod must be tuned on) -- : Notice: Undefined property: stdClass::$cart_id in /var/www/mysite/mytemplatefolder/mytemplate/html/com_hikashop/product/cart.php on line 221

Hi, as in subject: when I add products to cart I get empty cart message.
If I refresh page or move to another page the cart works and show all the products added.
With debug on I get only a notice in the cart, no errors in other pages.
Any solution please?
Thank you.

[EDIT]
I've disabled the legacy add to cart mode just to try and the problem still remain
Now, as my client ask, I've disabled the ajax add ti cart because we need a working site.
If you need I can give a temporary superadmin credential just to made some tests and solve my problem.
Thank you.