title: How to prevent collision of viewControllers?
body: <p>I have very usual navigation controller and I can handle local notification. When local notification is firing, new viewController is creating and showing. So it's very common situation, when I push and pop (back button) controllers in standard behaviour, when suddenly in push or pop  process, local notification is firing and new viewControllers is trying to appear and both fails. As the result - black screen, no responsive app. How to detect and prevent this collision? or should I use another approach?</p>