Failed to instantiate the default view controller for UIMainStoryboardFile

错误现象： XCODE 6.1 在使用storyboard时，报错：Failed to instantiate the default view controller for UIMainStoryboardFile 'MainStoryboard' - perhaps the designated entry point is not set?

原因分析：在StoryBoard中没有一个view controller设置了Initial Scene。

解决方案：在Storyboard中，选择一个view conroller作为story board的第一启动界面