error with very basic example - swift - Attempt to present on whose view is not in the window hierarchy

Question

I'm practicing with a basic example of alerts, it's two ViewControllers, each one has a button to go to the next or return, and another button to show an Alert

I added a Simple Alert in the ViewControler1 and it works fine, but if I add a Simple Alert in the ViewControler2 it does not work and shows the following error:

2018-09-12 16: 23: 43.107112-0500 proyect1 [74831: 1130476] Warning: Attempt to present on whose view is not in the window hierarchy!

Code viewController1:

import UIKit

class ViewController1: UIViewController {

    @IBAction func btnAlerta1 (_ sender: UIButton) {
        let alert = UIAlertController (style: .alert, title: "Verify your data", message: "Enter your email and password correctly")
        alert.addAction (title: "Ok", color: .black, style: .default) {action in}
        alert.show ()
    }
    override func viewDidLoad () {
        super.viewDidLoad ()
    }

}

Code viewController2:

import UIKit

class ViewController1: UIViewController {

    @IBAction func btnAlerta2 (_ sender: UIButton) {
        let alert = UIAlertController (style: .alert, title: "Verify your data", message: "Enter your email and password correctly")
        alert.addAction (title: "Ok", color: .black, style: .default) {action in}
        alert.show ()
    }
    override func viewDidLoad () {
        super.viewDidLoad ()
    }

}

And if I go back to the ViewController1 and wish to invoke the Simple Alert that worked before, now it does not work and it presents the same error!

could you help me?