From your other post, it seems that you're using Unity 5.4. I'm taking a wild guess and guessing that you're using STM with Unity UI. Unity UI didn't support multiple materials until Unity 2017.1, so quads are disabled in earlier versions. Sorry about that, but there just seemed to be no way around that in earlier versions, and it would always cause a crash.
I'm going to have to suggest either upgrading to 2017.1 or avoiding Unity UI. If you really wanted, you could set something up with custom events to place UI elements to imitate Quads, but that should just be a last resort.