Attribute Unavailable: Defines Presentation Context is not available prior to

  在xcode4.2的MainWindow.xib中，使用Tab Bar Controller或者Nav Controller时，
编译时候会有如下警告：
Attribute Unavailable: Defines Presentation Context is not available prior to Xcode 4.2

解决办法：
   Just uncheck the "Defines Context" checkbox in the attributes inspector. (Double-click on MainWindow.xib, select the navigation controller/Tab Bar Controller, then go to View->Utilities->Attributes Inspector.) That'll get rid of the warnings.