<Error>: CGContextSetFillColorWithColor

<Error>: CGContextSetFillColorWithColor: invalid context 0x0. This is a serious error. This application, or a library it uses, is using an invalid context  and is thereby contributing to an overall degradation of system stability and reliability. This notice is a courtesy: please fix this problem. It will become a fatal error in an upcoming update.

模拟器上调试不会出现这样的警告提示，真机一运行就出现这样的提示，多方查看后才知道这事iOS 6之后系统的问题，对运行没什么太大的影响，可以忽略，小小记录一下。