在这个PreferencesVC上，我有两个pickerViews供用户选择两种货币发送到主用户界面。在保存更改和发送数据时，它说意外地发现nil。为什么我的pickerViews总是有来自非空元组的数据？

protocol ChangeCurrencyDelegate {
    func userEnteredNewCurrency(top: CurrencyTuple, bottom: CurrencyTuple) 
}
class ExchangePreferencesVC: UIViewController {
  @IBOutlet weak var topCurrencyLabel: UILabel!
  @IBOutlet weak var topCurrencyPicker: UIPickerView!
  @IBOutlet weak var bottomCurrencyLabel: UILabel!
  @IBOutlet weak var bottomCurrencyPicker: UIPickerView!
  var delegate: ChangeCurrencyDelegate?
  var topPickerOptions = [(name: String, symbol: String, flag: UIImage)]()
  var bottomPickerOptions = [(name: String, symbol: String, flag: UIImage)]()
  var userCurrencyChoice: Currencies!
  override func viewDidLoad() {
    super.viewDidLoad()
    topPickerOptions = currencyISOCode
    bottomPickerOptions = currencyISOCode
  }
   // User top and bottom pickerView choices
  func createTopAndBottomCurrency() {
    let topUserIndex = topCurrencyPicker.selectedRow(inComponent: 0)
    let bottomUserIndex = bottomCurrencyPicker.selectedRow(inComponent: 0)
    let topUserChoice = currencyISOCode[topUserIndex]
    let bottomUserChoice = currencyISOCode[bottomUserIndex]
    userCurrencyChoice = Currencies(top: topUserChoice, bottom: bottomUserChoice)
  }
  // MARK: - Cancel and Save Changes actions
  @IBAction func saveChanges(_ sender: Any) {
    delegate?.userEnteredNewCurrency(top: userCurrencyChoice.top, bottom: userCurrencyChoice.bottom)
    self.dismiss(animated: true, completion: nil)
  }
}

字符串
委托是这样分配的->

// MARK: - Delegate of the ChangeCurrency protocol
extension ExchangeRateViewController: ChangeCurrencyDelegate {
  func userEnteredNewCurrency(top: CurrencyTuple, bottom: CurrencyTuple) {
    self.topFlag.setImage(top.flag, for: .normal)
    self.topSymbol.text = top.symbol
    self.bottomFlag.setImage(bottom.flag, for: .normal)
    self.bottomSymbol.text = bottom.symbol
  }
  override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
    if segue.identifier == "goToPreferences" {
      let destinationVC = segue.destination as! ExchangePreferencesVC
      destinationVC.delegate = self
    }
  }
}

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