An error occurred while attaching module (Dynamicweb.Frontend.Content)

System.NullReferenceException: Object reference not set to an instance of an object.
   at PriceGroupDiscountProvider.PriceGroupDiscountProviderExtender.DiscountValidForProduct(Product product)
   at Dynamicweb.Ecommerce.Orders.Discounts.DiscountProvider.GetValidDiscountsForProduct(IEnumerable`1 discounts, Product product, String languageId, Currency currency, String countryCode, Boolean ignoreOrderConditions, User user)
   at Dynamicweb.Ecommerce.Orders.Discounts.DiscountInfoCollection.LoadDiscounts(User user)
   at Dynamicweb.Ecommerce.Products.Product.GetDiscountMatrix(String currencyCode, String countryCode, User user, String unitId, String shopId)
   at Dynamicweb.Ecommerce.Frontend.Renderer.RenderProduct(Product product, Boolean extendedProperties, Template template, Int32 loopCounter, Boolean renderRelated, String productUrl, Int32 quantity, Int32 wishListId, Boolean renderPublicList, Boolean isRecursiveCall, Lazy`1 categoryFieldSorting, Int32 orderLineOfProductCount, RenderingContext renderingContext, String unitId)
   at Dynamicweb.Ecommerce.Frontend.Renderer.RenderProduct(Product product, Boolean extendedProperties, String unitId)
   at Dynamicweb.Ecommerce.Frontend.Frontend.RenderProduct(Product product, String unitId)
   at Dynamicweb.Ecommerce.Frontend.Frontend.EcomRender()
   at Dynamicweb.Ecommerce.Frontend.Frontend.GetContent()
   at Dynamicweb.Frontend.Content.GetModuleOutput(Paragraph paragraph, PageView pageview)