If a returning customer has entered a couponcode which no longer exists accessing, the cart can cause a crash

[AndreiMaz]

If a returning customer has entered a couponcode which no longer exists accessing, the cart can cause a crash due to the following lines in ShoppingCartModelFactory:

var discount = _discountService.GetAllDiscountsForCaching(couponCode: couponCode)
.FirstOrDefault(d => d.RequiresCouponCode && _discountService.ValidateDiscount(d, _workContext.CurrentCustomer).IsValid);

model.DiscountBox.AppliedDiscountsWithCodes.Add(new ShoppingCartModel.DiscountBoxModel.DiscountInfoModel()
{
Id = discount.Id,
CouponCode = discount.CouponCode
});

The discount in this case is null.

Source: http://www.nopcommerce.com/boards/t/47053/missing-null-check-causes-crash-in-cart.aspx