Swift app does not run on iOS 7

I am creating a application in swift for both iOS7 & iOS8 compatible. My app runs in iOS8 (as deployment target) easily for both simulator and device. But it's not running in iOS7 as deployment target under "Info" tab. For iOS7, if I choose iPhone 5s(8.1) simulator then it works, but if choose any simulator of 7.0/7.1 sdk/iPhone device, then my app crashes. What should I do? My base SDK on "build settings" tab in xcode is iOS 8.1. Below is my code. My app crashes on "UISearchController(searchResultsController: nil)" of below code. Any suggestion must be appreciable.

 override func viewDidLoad() {
        super.viewDidLoad()

        // Search Controller Setup
        searchController = UISearchController(searchResultsController: nil)
        searchController.searchBar.delegate = self
        searchController.searchResultsUpdater = self
        searchController.dimsBackgroundDuringPresentation = false
        searchController.searchBar.placeholder = "ব্র্যান্ড/সংস্থা/দোকান খুঁজুন"

        // Make sure the that the search bar is visible within the navigation bar.
        searchController.searchBar.sizeToFit()
        tableView.tableHeaderView = searchController.searchBar
        definesPresentationContext = true
    }

Solution

UISearchController is available from IOS8, if you want to use something like that in IOS7, you should use the UISearchDisplayController (which is deprecated in IOS8).