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

System.NullReferenceException: Object reference not set to an instance of an object.
   at Dynamicweb.Ecommerce.Products.ProductService.ReFactorProductList(ICollection`1 products, Currency currency, Country country, StockLocation stockLocation, User user, Boolean useAssortments)
   at Dynamicweb.Ecommerce.Products.ProductService.GetRelatedCountFrontend(Product product, Currency currency, Country country, StockLocation stockLocation, User user)
   at Dynamicweb.Ecommerce.Frontend.Renderer.GetRelatedCount(Product product, Boolean isFrontend, RenderingContext renderingContext)
   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.RenderProducts(ProductCollection products, Template parentTemplate, String loopName, Int32 loopCounter, Boolean renderRelatedProducts, Boolean renderRelatedLists, Boolean isRecursive, RenderingContext renderingContext)
   at Dynamicweb.Ecommerce.Frontend.Renderer.RenderProductsWithContext(ProductCollection products, Template parentTemplate, RenderingContext renderingContext)
   at Dynamicweb.Ecommerce.Frontend.Renderer.RenderProducts(ProductCollection products)
   at Dynamicweb.Ecommerce.Frontend.Frontend.ListProducts(ProductCollection products, String groupsIdent, Boolean hasBeenSorted)
   at Dynamicweb.Ecommerce.Frontend.Frontend.List(String groups)
   at Dynamicweb.Ecommerce.Frontend.Frontend.EcomRender()
   at Dynamicweb.Ecommerce.Frontend.Frontend.GetContent()
   at Dynamicweb.Frontend.Content.GetModuleOutput(Paragraph paragraph, PageView pageview)