Check the code that runs when you click on that button. You're getting an out of range exception. You can comment out the try/catch part around those lines and NGUI won't catch the exception anymore, so you will be able to jump directly to the offending line.
P.S. In the up to date version of NGUI it should already be like that. That entire section is inside a "if not in Unity editor" block. You should update.
