Losing reference to transitioningDelegate before presetentation presentation

I had a working custom UIPresentationController prior to Xcode beta 2 and iOS 10 beta 2. I haven't changed any code, but the presentation is now being presented with the standard modal presentation.

There is a note in the Apple sample code for UIPresentationController that says:

For presentations which will use a custom presentation controller, it is possible for that presentation controller to also be the transitioningDelegate. This avoids introducing another object or implementing in the source view controller.

transitioningDelegate does not hold a strong reference to its destination object.To prevent presentationController from being released prior to calling -presentViewController:animated:completion: the NS_VALID_UNTIL_END_OF_SCOPE attribute is appended to the declaration.

I've checked the transitioningDelegate on the presented view controller before and after the presentation. Before it is my custom UIPresentationController, but after it is nil. My guess is that the reference is being released, but I cannot find an equivalent to NS_VALID_UNTIL_END_OF_SCOPE in Swift. EDIT: I've verified that transitioningDelegate is set up until just before the presentation, and then is nil when it is time to present.

My code in the presenting view controller:

@IBAction func buttonAction(_ sender: UIButton) {
    let secondViewController = storyboard!.instantiateViewController(withIdentifier: "NewViewController") as! NewViewController
    let presentationController = MyPresentationController(presentedViewController: secondViewController, presenting: self)
    presentationController.initialFrame = button.frame
    secondViewController.transitioningDelegate = presentationController

    // Move map
    let pixelsToMove: CGFloat = mapView.frame.height / 4
    let region = self.mapView.region

    self.mapView.setRegion(region, offsetBy: pixelsToMove, animated: true)

    // Delegate to NewViewController
    secondViewController.mapView = mapView
    mapView.delegate = secondViewController

    print(secondViewController.transitioningDelegate)
    UIView.animate(withDuration: 0.3, animations: {
        let tabBar = self.tabBarController!.tabBar
        tabBar.frame.origin.y += tabBar.frame.height

        self.present(secondViewController, animated: true, completion: nil)
    })
}

And my code in the UIPresentationController:

override init(presentedViewController: UIViewController, presenting presentingViewController: UIViewController?) {
    super.init(presentedViewController: presentedViewController, presenting: presentingViewController)
    presentedViewController.modalPresentationStyle = .custom
}

The problem was in beta 2 the method signatures inUIViewControllerTransitioningDelegate changed, and so they were not being called in my code. I don't understand why, but once again everything works perfect without explicitly storing a strong reference to the presentation controller.