MacOS应用cocoa application问题
CGContextDrawPath: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
关于如何set环境变量：
Product -> Scheme -> Edit Scheme -> Run -> Arguments
在环境变量处添加CG_CONTEXT_SHOW_BACKTRACE并将值设置为Yes.
