Don't stop looking for the view controller when it's a UINavigationController

[markrickert]

I needed to put a UITabBarController inside a UINavigationController (I know, NOT recommended by Apple, but I had to do it nonetheless).

This fixes the stylesheet not being able to be set because when it found a UINavigationController as the root_view_controller, it just gave up. I'm asking the method to keep searching till it gets an actual UIViewController and not blindly trust that the visibleViewController is a UIViewController.

An alternative so that we don't have to recurse one more time would be something like this:

when UINavigationController
  if root_view_controller.visibleViewController.is_a?(UIViewController)
    root_view_controller.visibleViewController
  else
    current_view_controller(root_view_controller.visibleViewController)
  end
when UITabBarController
# ...