How could make call native Opengles rendering in script together with call Unity OnGUI show well in IOS

HI,

Now I am using Unity to call native opengles native code in script (call it in OnPostRender fun.) to make it run in IOS. I draw a triangle. It is good. And then, if I add Unity OnGUI() and then create Unity Button or some other GUI in script. At the first 1 second, it runs very good. But then, the iPad's screen become black, I can see nothing. Then it seems not so smooth. I am not sure what call this problem.

I have next questions: 1. Can I call native opengles native code in OnPostRender together with call Unity OnGUI （add Unity GUI）in IOS, then it will not come out black screen problem in iPad? 2. Is it because of the single-thread rendering in mobile device? I want to know what make this problem happened?