UINavigationController return nil Swift

When I try print(self.navigationController) in viewWillApear or viewDidLoad all ok. But when delegate return response from API print(self.navigationController) return nil. What could it be?

extension EnterpriseList: APIDataDelegate {
    func successRequest() { //print(self.navigtionController) == nil
        DispatchQueue.main.async {
            self.navigationController?.popToRootViewController(animated: true)
        }
    }
    
    func badRequest() {
        DispatchQueue.main.async {
            Alert.showWarningAlert(withTitle: "Внимание!", andMessage: "Ошибка получения данных, попробуйте чуть позже", whereSender: self)
        }
    }
    
}

class EnterpriseList: UIViewController {
     let dataSource = DataSource()

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

   extension EnterpriseList: APIDataDelegate {
      func successRequest() {
          DispatchQueue.main.async {
            self.navigationController!.popToRootViewController(animated: true)
        }
   }

   func badRequest() {
      DispatchQueue.main.async {
      }
   }
}

protocol APIDataDelegate: class {
    func successRequest()
    func badRequest()
}

class DataSource {
    var delegate: APIDataDelegate?

    private let queue = DispatchQueue(label: "Test", qos: .background, attributes: [], autoreleaseFrequency: .inherit, target: nil)

    func makeAPICall() {
        queue.asyncAfter(deadline: .now() + 2) {[weak self] in
            self?.delegate?.successRequest()
        }
        queue.asyncAfter(deadline: .now() + 10) {[weak self] in
            self?.delegate?.successRequest()
        }
    }
}

The crash is because you are calling the method on deallocated object.

Here is how to fix this: Make your delegate is weak and you are correctly deallocating the objects

weak var delegate: APIDataDelegate?