class ViewController: UIViewController, UITextFieldDelegate {

    @IBOutlet var textInputScreen: UITextField!

    override func viewDidLoad() {
       super.viewDidLoad()
       textInputScreen.delegate = self
    }

    func textFieldShouldReturn(_ textField: UITextField) -> Bool {
       // textField.canResignFirstResponder is true
       textField.resignFirstResponder() // throws error
       // view.endEditing(true) // throws same error
       // textInputScreen.resignFirstResponder() // throws same error
       return true
    }