Fix unbalanced called to begin/end appearance selectors

This is to fix a case I was seeing where we were calling `beginAppearanceTransition:animated` while `self.currentContentViewController` was still `nil`, and then calling `endAppearanceTransition` when it was no longer `nil`. This led to unbalanced calls.
You can probably make this a `UIViewController *` instead of `id` and then shorten to variable name to loose the "viewController..." part of it.