UIViewController deallocated when using function replaceViewController(with:completion)

## What did you do?

I'm trying to replace the view controller on the main window of the application, but in commit https://github.com/HeroTransitions/Hero/commit/800b9d90238e385df8bb69f5e94887e2d0e7a695 you've introduced a regression. by removing `!` from `#if !TARGET_IS_EXTENSION`. Probably it is also causing issues in extensions that do not have `UIApplication` class. Actually, that code was removed in favor of code from PR #705:
```
if let parentVC = parentVC {
base.dismiss(animated: false) {
parentVC.present(next, animated: false, completion: completion)
}
} else {
parentVC?.view.window?.rootViewController = next
}
```
which doesn't have any sense because in else statement `parentVC` will be always `nil`.

## What did you expect to happen?

It should just replace the root view controller on that window without deallocating view controller.

## What happened instead?

View in the hierarchy is replaced but View Controller is deallocated.

## General Information

* Hero Version: 1.6.0

* iOS Version(s): All

* Swift Version: All

* Devices/Simulators: All

* Reproducible in Examples? (Yes/No):
Reproducible in any project when you want to replace root view in any `UIWindow`