Error- Object reference not set to an instance of an object - When Open Cart page.

var discount = _discountService.GetAllDiscountsForCaching(couponCode: couponCode).FirstOrDefault(d => d.RequiresCouponCode && _discountService.ValidateDiscount(d, _workContext.CurrentCustomer).IsValid);
 
if(discount != null)
{
   model.DiscountBox.AppliedDiscountsWithCodes.Add(new ShoppingCartModel.DiscountBoxModel.DiscountInfoModel()
   {
      Id = discount.Id,
      CouponCode = discount.CouponCode
   });
}