Error message: Object reference not set to an instance of an object.

Url: https://www.lonas.gr/skin.aspx?lang=el&itemtype=Product&itemid=0000221A&action=view&

Message: Object reference not set to an instance of an object.

   at ASP.tpl_product_view_tpl_ascx.<__Render__control1>b__d(ProductCategoryProduct c) in c:\inetpub\vhosts\lonas.gr\tpl\Product.View.tpl.ascx:line 402
   at System.Linq.Enumerable.WhereSelectListIterator`2.MoveNext()
   at System.String.Join(String separator, IEnumerable`1 values)
   at ASP.tpl_product_view_tpl_ascx.__Render__control1(HtmlTextWriter __w, Control parameterContainer) in c:\inetpub\vhosts\lonas.gr\tpl\Product.View.tpl.ascx:line 402
   at System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children)
   at System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter)
   at Bone.ActionView.Render(TextWriter w) in C:\Bone\Bone\src\Actions\DBActions\ActionView.cs:line 161
   at Bone.Action.RenderContent(TextWriter w) in C:\Bone\Bone\src\Actions\_Action.cs:line 394
   at Bone.Action.ToString() in C:\Bone\Bone\src\Actions\_Action.cs:line 518
   at Bone.Base.get_Content() in C:\Bone\Bone\src\Classes\Base.cs:line 80
   at ASP.skin_aspx.__Render__control1(HtmlTextWriter __w, Control parameterContainer) in c:\inetpub\vhosts\lonas.gr\skin.aspx:line 216
   at System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children)
   at System.Web.UI.Page.Render(HtmlTextWriter writer)
   at Bone.Asp.RenderSkin(TextWriter w) in C:\Bone\Bone\src\Classes\Asp.cs:line 71
   at Bone.ModalAction.RenderPage(TextWriter w) in C:\Bone\Bone\src\Actions\Modal.cs:line 81
   at Bone.Asp.Render(HtmlTextWriter w) in C:\Bone\Bone\src\Classes\Asp.cs:line 82
   at System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter)
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)