UIApplicationDelegate's window: UIWindow? is optional. This conformance is only required if you specify storyboard in Info.plist (because UIKit would need to do something like delegate.window?.someLogicToInflateMyStorybardAndPutTheEntryViewControllerOntoTheWindow())

In our implementation, the window property isn't optional.

Note that changing window to optional will be a breaking change. I suspect this API isn't commonly used, but we need to think about the implication of the breaking change.