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

System.NullReferenceException: Object reference not set to an instance of an object.
   at Lucene.Net.Index.SegmentTermDocs..ctor(SegmentReader parent)
   at Lucene.Net.Index.SegmentReader.TermDocs()
   at Lucene.Net.Search.FieldCacheImpl.StringCache.CreateValue(IndexReader reader, Entry entryKey)
   at Lucene.Net.Search.FieldCacheImpl.Cache.Get(IndexReader reader, Entry key)
   at Lucene.Net.Search.FieldCacheImpl.GetStrings(IndexReader reader, String field)
   at Dynamicweb.Indexing.Lucene.LuceneIndexProvider.CaseIgonreComparator.SetNextReader(IndexReader reader, Int32 docBase)
   at Lucene.Net.Search.TopFieldCollector.MultiComparatorNonScoringCollector.SetNextReader(IndexReader reader, Int32 docBase)
   at Lucene.Net.Search.IndexSearcher.Search(Weight weight, Filter filter, Collector collector)
   at Lucene.Net.Search.IndexSearcher.Search(Weight weight, Filter filter, Int32 nDocs, Sort sort, Boolean fillFields)
   at Lucene.Net.Search.IndexSearcher.Search(Weight weight, Filter filter, Int32 nDocs, Sort sort)
   at Dynamicweb.Indexing.Lucene.LuceneIndexProvider.SearchInternal(IQuery query, QuerySettings settings)
   at Dynamicweb.Indexing.Querying.QueryService.Query(IQuery query, QuerySettings settings)
   at Dynamicweb.Ecommerce.Indexing.ProductQueryHelper.ExecuteQuery(IQuery query, QuerySettings settings)
   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)
   at Dynamicweb.Ecommerce.ProductCatalog.ProductCatalogFrontend.GetContent()
   at Dynamicweb.Frontend.Content.GetModuleOutput(Paragraph paragraph, PageView pageview)