Set alertWindow.rootViewController = nil

In the demo, when calling ```show5StackedAlertViews:```, ```self.alertWindow``` is remaining, even though it is set to nil, and is blocking the main window. I was able to confirm this using Reveal. Setting ```alertWindow.rootViewController``` to nil seems to clear this up.

Out of curiosity, would this be due to something holding a reference to ```alertWindow```, and not allowing ARC to release it? I'm still trying to get a handle on all this, and while I was able to make it work, I want to make sure I correctly understand *how* I made it work.