Swift初心者です。
このたび、iOSのSwiftを使って簡単なブラウザを作ってみたのですが、AppDelegate.swiftの

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate 

の行で以下のようなエラー文が発生していました。

Thread 1:signal SIGABRT 

原因は何でしょうか？
下にviewController.swiftのソースコードを載せておきます。Xcodeはversion 9.0 beta、Swift3です。

import UIKit

class ViewController: UIViewController {

    @IBOutlet weak var urlTextField: UITextField!
    @IBOutlet weak var browserWebView: UIWebView!

    @IBOutlet weak var backButton: UIBarButtonItem!
    @IBOutlet weak var forwardButton: UIBarButtonItem!
    @IBOutlet weak var reloadButton: UIBarButtonItem!

    override func viewDidLoad() {
        super.viewDidLoad()

        let urlString = "http://dotinstall.com"
        let urlRequest = URLRequest(url: URL(string: urlString)!)
        self.browserWebView.loadRequest(urlRequest)

        // Do any additional setup after loading the view, typically from a nib.
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }

    @IBAction func goBack(_ sender: Any) {
    }

    @IBAction func goForward(_ sender: Any) {
    }

    @IBAction func reload(_ sender: Any) {
    }
}

以下のようなエラーが発生します。

2017-07-07 11:10:19.864054+0900 MyBrowserApp[74215:10111362] [Snapshotting] Snapshotting a view (0x7ffdade10c70, _UIButtonBarStackView) that has not been rendered at least once requires afterScreenUpdates:YES.
2017-07-07 11:10:22.310946+0900 MyBrowserApp[74215:10111675] [] network_config_register_boringssl_log_debug_updates Failed to register for BoringSSL log debug updates
2017-07-07 11:10:24.343312+0900 MyBrowserApp[74215:10111688] WF: === Starting WebFilter logging for process MyBrowserApp
2017-07-07 11:10:24.343518+0900 MyBrowserApp[74215:10111688] WF: _userSettingsForUser : (null)
2017-07-07 11:10:24.343758+0900 MyBrowserApp[74215:10111688] WF: _WebFilterIsActive returning: NO
2017-07-07 11:10:25.109372+0900 MyBrowserApp[74215:10111682] [] network_config_register_boringssl_log_debug_updates Failed to register for BoringSSL log debug updates
2017-07-07 11:10:25.334517+0900 MyBrowserApp[74215:10111677] [] network_config_register_boringssl_log_debug_updates Failed to register for BoringSSL log debug updates
2017-07-07 11:10:25.445752+0900 MyBrowserApp[74215:10111677] [] network_config_register_boringssl_log_debug_updates Failed to register for BoringSSL log debug updates
2017-07-07 11:10:25.553913+0900 MyBrowserApp[74215:10111676] [] network_config_register_boringssl_log_debug_updates Failed to register for BoringSSL log debug updates