Lỗi ở file demo.swift

Demo.swift

import UIKit

class ViewController: UIViewController {

    @IBOutlet weak var txtField: UITextField!
    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
    }

    @IBAction func onSayHello() {
        //print("Hello World" + txtField.text!)
        let n = 5
        for i in 1...n {
            print(i)
        }
    }
}