Missing Null check causes crash in cart

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
                });