Failed when present UIviewcontroller - uiviewcontroller

Question

I have 2 viewcontrollers: mainVC & listVC.
I tapped customized button on mainVC to present listVC, then tapped button on listVC to go back to mainVC. until now there is no any failure. But when I tap customized button to go to listVC again, program failed.
error:
NSInvalidArgumentException', reason: 'Application tried to present modal view controller on itself. Presenting controller is
Voice_of_Animals.ListVC: 0x7feeee513210>
class ViewController: UIViewController
{
var cb = CirButton(Circolor: UIColor.cyan, Rsize: 100, PositionX: 100, PositionY: 100)
override func viewDidLoad() {
super.viewDidLoad()
mainVC = self
self.view.addSubview(cb)
listVC.transitioningDelegate = listVC as UIViewControllerTransitioningDelegate
}
func showNextPage() {
self.present(listVC, animated: true, completion: nil)
}
#IBAction func show(_ sender: UIButton) {
self.present(listVC, animated: true, completion: nil)
}
#IBAction func Touch(_ sender: CButton) {
sender.RunAnimation()
}
override func didReceiveMemoryWarning() {
}
}
class ListVC: ViewController{
var transition = FadeAnimator()
var btn1 = CirButton(Circolor: UIColor.cyan, Rsize: 100, PositionX: 100, PositionY: 100)
#IBOutlet weak var myList: ListView!
#IBAction func Push(_ sender: UIButton) {
}
#IBAction func edgeSlide(_ sender: UIScreenEdgePanGestureRecognizer) {
mainVC.dismiss(animated: true, completion: nil)
}
override func viewDidLoad() {
super.viewDidLoad()
self.view.addSubview(btn1)
}
}