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.