k. I don't even know what you wane use this for but hey.. that's your problem.
anyway your error is because you trying to use the "new function" for something its not made for. (you have no return value.) about "New Function" read this: http://forums.sc2mapster.com/resources/tutorials/1076-trigger-utilizing-the-power-of-functions-with-gui/
anyway think your better off using "new action definition" for what your trying to do.