fatal error: unexpectedly found nil while unwrapping an Optional value

var page: Page?

let artWork = UIImageView()
let storyLabel = UILabel()
let firstChoiceButton = UIButton(type: .System)
let secondChoiceButton = UIButton(type: .System)


required init?(coder aDecoder: NSCoder) {
    super.init(coder:aDecoder)
}

init(page:Page){

    self.page = page
    super.init(nibName:nil,bundle:nil)
}


override func viewDidLoad() {
    super.viewDidLoad()

    view.backgroundColor = .whiteColor()





    if let page = page {
        artWork.image = page.story.artWork


        let attributeString = NSMutableAttributedString(string: page.story.text)

        let paragraphStyle = NSMutableParagraphStyle()
        paragraphStyle.lineSpacing = 10

        attributeString.addAttribute(NSParagraphStyleAttributeName, value: paragraphStyle, range: NSMakeRange(0, attributeString.length))

        storyLabel.attributedText = attributeString

        if let firstChoice = page.firstChoice {
            firstChoiceButton.setTitle(firstChoice.title, forState: .Normal)
            firstChoiceButton.addTarget(self, action: #selector(PageController.loadFirstChoice), forControlEvents: .TouchUpInside)
        } else {

            firstChoiceButton.setTitle("play again", forState: .Normal)
            firstChoiceButton.addTarget(self, action: #selector(PageController.playAgain), forControlEvents: .TouchUpInside)
        }

            if let secondChoice = page.secondChoice {
                secondChoiceButton.setTitle(secondChoice.title, forState: .Normal)



            }
        }


    }


override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()

}

override func viewWillLayoutSubviews() {
    view.addSubview(artWork)
    artWork.translatesAutoresizingMaskIntoConstraints = false

    NSLayoutConstraint.activateConstraints([
        artWork.topAnchor.constraintEqualToAnchor(view.topAnchor),
        artWork.bottomAnchor.constraintEqualToAnchor(view.bottomAnchor),
        artWork.rightAnchor.constraintEqualToAnchor(view.rightAnchor),
        artWork.leftAnchor.constraintEqualToAnchor(view.leftAnchor)])

    view.addSubview(storyLabel)
    storyLabel.translatesAutoresizingMaskIntoConstraints = false
    storyLabel.numberOfLines = 0

  NSLayoutConstraint.activateConstraints([
    storyLabel.leadingAnchor.constraintEqualToAnchor(view.leadingAnchor, constant: 16.0),
    storyLabel.trailingAnchor.constraintEqualToAnchor(view.trailingAnchor, constant: -16.0),
    storyLabel.topAnchor.constraintEqualToAnchor(view.centerYAnchor, constant: -48.0)])

    view.addSubview(firstChoiceButton)
    firstChoiceButton.translatesAutoresizingMaskIntoConstraints = false
    NSLayoutConstraint.activateConstraints([
        firstChoiceButton.centerXAnchor.constraintEqualToAnchor(view.centerXAnchor),
        firstChoiceButton.bottomAnchor.constraintEqualToAnchor(view.bottomAnchor, constant: -80.0)
        ])

    view.addSubview(secondChoiceButton)
    secondChoiceButton.translatesAutoresizingMaskIntoConstraints = false
    NSLayoutConstraint.activateConstraints([
        secondChoiceButton.centerXAnchor.constraintEqualToAnchor(view.centerXAnchor),
        secondChoiceButton.bottomAnchor.constraintEqualToAnchor(view.bottomAnchor, constant: -32)
        ])

}

func loadFirstChoice() {

    if let page = page,firstChoice = page.firstChoice {
        let nextPage = firstChoice.page
        let pageController = PageController(page:nextPage)

        navigationController?.pushViewController(pageController, animated: true)


    }
}

func loadSecondChoice() {

    if let page = page,secondChoice = page.secondChoice {
        let nextPage = secondChoice.page
        let pageController = PageController(page:nextPage)

        navigationController?.pushViewController(pageController, animated: true)

}

    navigationController?.popToRootViewControllerAnimated(true)
}


}

var artWork: UIImage {


    return UIImage(named: self.rawValue)!


}




var text: String {
    switch self {
    case .ReturnTrip:
        return "On your return trip from studying Saturn's rings, you hear a distress signal that seems to be coming from the surface of Mars. It's strange because there hasn't been a colony there in years. \"Help me, you're my only hope.\""
    case .TouchDown:
        return "You deftly land your ship near where the distress signal originated. You didn't notice anything strange on your fly-by, behind you is an abandoned rover from the early 21st century and a small crate."
    case .Homeward:
        return "You continue your course to Earth. Two days later, you receive a transmission from HQ saing that they have detected some sort of anomaly on the surface of Mars near an abandoned rover. They ask you to investigate, but ultimately the decision is yours because your mission has already run much longer than planned and supplies are low."
    case .Rover:
        return "The rover is covered in dust and most of the solar panels are broken. But you are quite surprised to see the on-board system booted up and running. In fact, there is a message on the screen. \"Come to 28.2342, -81.08273\". These coordinates aren't far but you don't know if your oxygen will last there and back."
    case .Cave:
        return "Your EVA suit is equipped with a headlamp which you use to navigate to a cave. After searching for a while your oxygen levels are starting to get pretty low. You know you should go refill your tank, but there's a faint light up ahead."
    case .Crate:
        return "Unlike everything else around you the crate seems new and...alien. As you examine the create you notice something glinting on the ground beside it. Aha, a key! It must be for the crate..."
    case .Monster:
        return "You pick up the key and try to unlock the crate, but the key breaks off in the keyhole.You scream out in frustration! Your scream alerts a creature that captures you and takes you away..."
    case .Droid:
        return "After a long walk slightly uphill, you end up at the top of a small crater. You look around and are overjoyed to see your robot friend, Droid-S1124. It had been lost on a previous mission to Mars. You take it back to your ship and fly back to Earth."
    case .Home:
        return "You arrive home on Earth. While your mission was a success, you forever wonder what was sending that signal. Perhaps a future mission will be able to investigate."
    }
}

let story: Story

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

var firstChoice: Choice?
var secondChoice: Choice?

init(story: Story) {

self.story = story
}

func addChoice(title: String,story:Story)->Page{

    let page = Page(story: story)

    return addChoice(title, page: page)
}

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

    switch (firstChoice,secondChoice) {
    case (.Some,.Some): break
    case (.None,.None),(.None,.Some): firstChoice = (title,page)
    case (.Some,.None): secondChoice = (title,page)
    }
    return page
}

static var story: Page {

    let returnTrip = Page(story: .ReturnTrip)
    let touchdown = returnTrip.addChoice("Stop and Investigate", story: .TouchDown)
    let homeward = returnTrip.addChoice("Continue Home to Earth", story: .Homeward)
    let rover = touchdown.addChoice("Explore the Rover", story: .Rover)
    let crate = touchdown.addChoice("Open the Crate", story: .Crate)

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

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

    cave.addChoice("Continue towards faint light", story: .Droid)
    cave.addChoice("Refill the ship and explore the rover", page: rover)

    crate.addChoice("Explore the Rover", page: rover)
    crate.addChoice("Use the key", story: .Monster)

    return returnTrip

}