UIPickerView display question mark

I'm trying to display Blood type in PickerView, but for some reason the choices are displayed as question marks only !

Here is my code:

    class RegisterViewController: UIViewController, UIPickerViewDataSource, UIPickerViewDelegate {    //Methods for PickerViewDataSource    @available(iOS 2.0, *)    public func numberOfComponents(in pickerView: UIPickerView) -> Int {        return 1    }    func pickerView(_ pickerView: UIPickerView, numberOfRowsInComponent component: Int) -> Int {        return pickerDataSource.count;    }    func pickerView(pickerView: UIPickerView, titleForRow row: Int, forComponent component: Int) -> String! {        return pickerDataSource[row]    }    @IBOutlet weak var selectedDate: UILabel!    @IBOutlet weak var birthDatePicker: UIDatePicker!    @IBOutlet weak var mobileNumberField: UITextField!    @IBOutlet weak var emailField: UITextField!    @IBOutlet weak var nameField: UITextField!    @IBOutlet weak var bloodTypePicker: UIPickerView!    @IBOutlet weak var selectedType: UILabel!    let pickerDataSource = ["Group A", "Group B", "Group AB", "Group O"];    override func viewDidLoad() {        super.viewDidLoad()        // Do any additional setup after loading the view.        super.viewDidLoad()        self.bloodTypePicker.dataSource = self;        self.bloodTypePicker.delegate = self;    }    func pickerView(pickerView: UIPickerView, didSelectRow row: Int, inComponent component: Int)    {        if(row == 0)        {            self.selectedType.text = "Group A"        }        else if(row == 1)        {            self.selectedType.text = "Group B"        }        else if(row == 2)        {            self.selectedType.text = "Group AB"        }        else        {            self.selectedType.text = "Group O"        }    }    override func didReceiveMemoryWarning() {        super.didReceiveMemoryWarning()        // Dispose of any resources that can be recreated.    }    @IBAction func datePickerAction(_ sender: Any) {        var dateFormatter = DateFormatter()        dateFormatter.dateFormat = "dd-MM-yyyy"        var strDate = dateFormatter.string(from: birthDatePicker.date)        self.selectedDate.text = strDate    }}

and here is a screenshot of what is displayed:

I was following this tutorial: http://sourcefreeze.com/ios-uipickerview-example-using-swift/

Thanks in advance.