UISearchDisplayController creates and manages the navigation item needed to display the search bar in the navigation bar. You don't need to create your own, although you can access it via searchDisplayController.navigationItem
after displaysSearchBarInNavigationBar
has been set to YES (the navigationItem is created lazily)
When the view controller with the search display controller is asked for its navigationItem, the search display controller will return its item (which contains the search bar) instead of the view controller's.