Error in cart when trying to pay remaining balance fails
If I try to pay a remaining balance of an order (regardless if anything has been paid for this order at all or not) the order is placed into a (NullCart) cart and I'm sent to checkout.
If I cancel this checkout (or payment fails somehow) the order remains in the cart and I can't remove it.
Method "remove" in satchmo_store/shop/views/cart.py calls "_set_quantity" which returns "No cart to update." cause it's a NullCart.