ViewData["Category"] = _businessLayer.ViewAllCategory().ToString();
ViewData["Category"] is being set to a string.
ViewData["Category"] as List<fashionHub_new.Models.category>)
the "as" tries to convert ViewData["Category"] (a string) as a List<category> and if it can't it returns null. As your ViewData is a string it will return null. You probably need to do this instead in your controller
ViewData["Category"] = _businessLayer.ViewAllCategory().ToList();