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

System.NullReferenceException: Object reference not set to an instance of an object.
   at DwCustomModules.CustomModules.ProductFilter.Frontend.<>c.b__4_12(Group n)
   at System.Linq.EnumerableSorter`2.ComputeKeys(TElement[] elements, Int32 count)
   at System.Linq.EnumerableSorter`1.Sort(TElement[] elements, Int32 count)
   at System.Linq.OrderedEnumerable`1.d__1.MoveNext()
   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at DwCustomModules.CustomModules.ProductFilter.Frontend.GetTemplateTags(Group selectedGroup, ParagraphCollection pageParagraphCollection, Template template)
   at DwCustomModules.CustomModules.ProductFilter.Frontend.GetContent()
   at Dynamicweb.Frontend.Content.GetModule(DataRow& ParagraphRow)