You cannot have variables or arrays in GUI event blocks, only via Trigger - Add Event To Trigger as Cakemaster mentioned. That is because you do not actually register the variable but the current value behind the variable, so if you change the contents of Timer/TimerIndex afterwards, that does not affect the event, it still refers to the timer object that it was in the beginning. If you want to stay GUI, you have to register all possible timers once and then filter via conditions.