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

System.NullReferenceException: Object reference not set to an instance of an object.
   at Dynamicweb.Ecommerce.Indexing.ValueMappers.ProductCategoryFieldValueMapper.MapValues(String fieldId, IEnumerable`1 terms)
   at Dynamicweb.Indexing.Querying.ValueMapperBase.GetMappingsInternal(String fieldId, IEnumerable`1 terms, Boolean includeOccurrences)
   at Dynamicweb.Ecommerce.Indexing.ProductQueryHelper.HandleFieldFacets(Facet facet, FacetGroupResult facets, IIndex index, IQuery query, IList`1 facetResults, Object value, IEnumerable`1 valueMappers)
   at Dynamicweb.Ecommerce.Indexing.ProductQueryHelper.HandleFacetGroupResult(IQuery query, IFacetGroup facetGroup, FacetGroupResult facetGroupResult, IDictionary`2 parameters)
   at Dynamicweb.Ecommerce.Indexing.ProductQueryHelper.HandleFacetGroupResult(IQuery query, IEnumerable`1 facetGroups, IDictionary`2 parameters, IQueryResult result)
   at Dynamicweb.Ecommerce.Indexing.ProductQueryHelper.GetProductsAutoIdsFromIndexQuery(IQuery query, ProductCatalogSettings settings, QuerySettings querySettings, IEnumerable`1& facetViewModel, IEnumerable`1& spellCheckerResult)
   at Dynamicweb.Ecommerce.ProductCatalog.ProductCatalogFrontend.RenderProductList(ProductCatalogSettings settings, QuerySettings querySettings, IQuery query, String groupId, Boolean feed, Boolean compare)
   at Dynamicweb.Ecommerce.ProductCatalog.ProductCatalogFrontend.GetModuleContent()
   at Dynamicweb.Frontend.Content.GetModuleOutput(Paragraph paragraph, PageView pageview)