Change navigation back button image

override func viewDidLoad() {
    super.viewDidLoad()

    let backBtn = UIBarButtonItem(title: "", style: .plain, target: nil, action: nil)
    backBtn.setBackButtonBackgroundImage(UIImage(named:"myBackButton"), for: .normal, barMetrics: .default)
    navigationItem.backBarButtonItem = backBtn
    navigationController?.navigationBar.backIndicatorImage = UIImage()
    navigationController?.navigationBar.backIndicatorTransitionMaskImage = UIImage()

}