Alert box is not displaying

import UIKit

class ViewController: UIViewController {

    @IBOutlet weak var nameTextField: UITextField!

    override func viewDidLoad() {
        super.viewDidLoad()

        NotificationCenter.default.addObserver(self, selector: #selector(ViewController.keyboardWillShow(_:)), name: Notification.Name.UIKeyboardWillShow, object: nil)

    }

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

    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
        if segue.identifier == "startAdventure" {

            do {
                if let name = nameTextField.text {
                    if name == "" {
                        throw AdventureError.nameNotProvided
                    } else {
                        guard let pageController = segue.destination as? PageController else { return }

                        pageController.page = Adventure.story(withName: name)
                    }
                }
            } catch AdventureError.nameNotProvided {
                let alertController = UIAlertController(title: "Name Not Provided", message: "Provide a name to start the story", preferredStyle: .alert)

                let action = UIAlertAction(title: "OK", style: .default, handler: nil)
                alertController.addAction(action)

                present(alertController, animated: true, completion: nil)
            } catch let error {
                fatalError("\(error.localizedDescription)")
            }
        }
    }

    func keyboardWillShow(_ notification: Notification){
        print("Keyboard will show")
    }


    deinit{
        NotificationCenter.default.removeObserver(self)
    }
}

import Foundation

enum AdventureError: Error{
    case nameNotProvided
}

class Page{
    let story: Story

    typealias Choice = (title: String, page: Page)

    var firstChoice: Choice?
    var secondChoice: Choice?

    init(story: Story){
        self.story = story
    }
}

extension Page{


    func addChoiceWith(title: String, story: Story) -> Page {
        let page = Page(story: story)
        return addChoiceWith(title: title, page: page)
    }

    func addChoiceWith(title: String, page: Page) -> Page{

        switch (firstChoice, secondChoice) {
        case (.some, .some): return self
        case (.none, .none), (.none, .some): firstChoice = (title, page)
        case (.some, .none): secondChoice = (title, page)
        }

        return page
    }
}

struct Adventure {
    static func story(withName name: String) -> Page {
    let returnTrip = Page(story: .returnTrip(name: name))
        let touchdown = returnTrip.addChoiceWith(title: "Stop and Investigate", story: .touchDown)
        let homeward = returnTrip.addChoiceWith(title: "Continue home to Earth", story: .homeward)
        let rover = touchdown.addChoiceWith(title: "Explore the Rover", story: .rover(name: name))
        let crate = touchdown.addChoiceWith(title: "Open the Crate", story: .crate)

        homeward.addChoiceWith(title: "Head back to Mars", page: touchdown)
        let home = homeward.addChoiceWith(title: "Continue Home to Earth", story: .home)

        let cave = rover.addChoiceWith(title: "Explore the Coordinates", story: .cave)
        rover.addChoiceWith(title: "Return to Earth", page: home)

        cave.addChoiceWith(title: "Continue towards faint light", story: .droid(name: name))
        cave.addChoiceWith(title: "Refill the ship and explore the rover", page: rover)

        crate.addChoiceWith(title: "Explore the Rover", page: rover)
        crate.addChoiceWith(title: "Use the key", story: .monster)

        return returnTrip
    }
}