facebook/three20 — pull request #754 by kemenaran (2012-01-20 13:35:56+00:00)
Title: Safeguard against UITabBarController selecting  a controller not in the tab bar

Apple Docs states that only controllers in -[UITabBarController viewControllers] can be set as the -selectedViewController.

If for some reasons iOS messes up the view controllers hierarchy (it happens), setting an unknown view controller will fail (and present a blank screen). This fix avoids that.