title: App crashes when i tap iphone back button
body: <p>Hello World:) I have a rate button in my game, which goes to appstore when I click it. But if I tap little back button above the iphone, my app crashes. This code I have, when I go to appstore app:</p>
  
  <pre><code>internal func rate(appId: String, completion: @escaping ((_ success: Bool) -&gt; ())) {
      guard let url = URL(string: "itms-apps://itunes.apple.com") else {
          completion(false)
          return
      }
      guard #available(iOS 10, *) else {
          completion(UIApplication.shared.openURL(url))
          return
      }
      UIApplication.shared.open(url, options: [:], completionHandler: completion)
  }
  </code></pre>
  
  <p>in completionHandler I have:</p>
  
  <pre><code>internal func rateUsButtonTouched() {
      isUserInteractionEnabled = false
      menuSceneNodeDelegate?.rate(appId: "appID", completion: {_ in 
          self.isUserInteractionEnabled = true
      })
  }
  </code></pre>
  
  <p>These both of codes go in delegate. Can't have any idea what problem is!</p>