I am new to Swift, and I am trying to develop a quiz app. When I try to load the app on my iPhone, i get this error of SIGABRT in the AppDelegate class, and then there is just a white screen on the phone. How can I fix this? I have already tried to clean the code, and I erased and checked carefully the links that i have between the buttons and the code (as suggested in another question with those answers), and I still get this error. 

below is my view class

```
import UIKit
import Foundation

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        updateQuestion()
    }

    //Label
    @IBOutlet weak var label: UILabel!
    @IBOutlet weak var scorelabel: UILabel!
    @IBOutlet weak var flagview: UIImageView!

    //Botones
    @IBOutlet var botona: UIButton!

    @IBOutlet var botonb: UIButton!

    @IBOutlet var botonc: UIButton!

    @IBOutlet var botond: UIButton!

    let allQuestions = QuestionBank()
    var questionNumber: Int = 0
    var score: Int = 0
    var selectedAnswer: Int = 0

    //Button
    @IBAction func boton(_ sender: UIButton) {
        if sender.tag == selectedAnswer {
            print("Correcto")
            score+=1
        }
        else {
         print("Incorrecto")
        }
        updateQuestion()
    }

    func updateQuestion(){
    flagview.image=UIImage(named:allQuestions.list[questionNumber].questionImage)
        label.text=allQuestions.list[questionNumber].question

        botona.setTitle(allQuestions.list[questionNumber].optionA, for: UIControlState.normal)

        botonb.setTitle(allQuestions.list[questionNumber].optionB, for: UIControlState.normal)

        botonc.setTitle(allQuestions.list[questionNumber].optionC, for: UIControlState.normal)

        botond.setTitle(allQuestions.list[questionNumber].optionD, for: UIControlState.normal)

        selectedAnswer = allQuestions.list[questionNumber].correctAnswer

        questionNumber += 1
    }

    func updateUI(){

    }

    func restartQuiz(){

    }

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

```

Any help would REALLY be appreciated! Thank you in advance.