Hi,
You're displaying the view "product / cart".
Your problem is related to the variable "cartHelper" which should be an object but which is currently "null" in your case.
If we take a look at the "view.html.php" file for "product", we can see that code in the function "cart" (function called just before the display of the view), line 2216:
$cartHelper = hikashop_get('helper.cart');
$this->assignRef('cartHelper', $cartHelper);
$this->cart = $this->cartHelper;
So the cart helper is right loaded by HikaShop and right assigned as reference for the view.
Now, the first possibility is that your helper cart file is missing.
The second possibility is that you have something in your website which interact with HikaShop and remove that reference.
Regards,