EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0)

var score = 0
for question in QuestionController.questionsList {
    score += question.selectedAnswerIndex!

Full code for that swift file

class ResultsController: UIViewController {

  let resultsLabel: UILabel = {
    let label = UILabel()
    label.text = "Congratulations! You'd make a great Ross!"
    label.contentMode = .scaleToFill
    label.numberOfLines = 0
    label.translatesAutoresizingMaskIntoConstraints = false
    label.textAlignment = .center
    label.font = UIFont.boldSystemFont(ofSize: 30)
    return label
  }()

  override func viewDidLoad() {
    super.viewDidLoad()

    navigationItem.rightBarButtonItem = UIBarButtonItem(title: "Done", style: .plain, target: self, action: #selector(done(sender:)))

    navigationItem.title = "Results"

    view.backgroundColor = UIColor.white

    view.addSubview(resultsLabel)
    view.addConstraints(NSLayoutConstraint.constraints(withVisualFormat: "H:|[v0]|", options: NSLayoutFormatOptions(), metrics: nil, views: ["v0": resultsLabel]))
    view.addConstraints(NSLayoutConstraint.constraints(withVisualFormat: "V:|[v0]|", options: NSLayoutFormatOptions(), metrics: nil, views: ["v0": resultsLabel]))

    let names = ["Ross", "Joey", "Chandler", "Monica", "Rachel", "Phoebe"]

    var score = 0
    for question in QuestionController.questionsList {
      score += question.selectedAnswerIndex!
    }

    let result = names[score % names.count]
    resultsLabel.text = "Congratulations! \(result)."
  }

  @objc func done(sender: UIBarButtonItem) {

    navigationController?.popToRootViewController(animated: true)

  }

}