Personally, I would find list_box / popup_list tiles easier to manipulate for this task, though radio_button tiles could also be used.
If you want to proceed with radio_button tiles, I would suggest setting a variable within the action_tile expression for each radio_button, then checking this single variable after the user has exited the dialog.
Hence the action_tile statement might be something like:
- (action_tile "gate_radio_button_key" "(setq choice "gate")")
This is the very reason that I state that list_box / popup_list tiles would be easier, since with radio_button tiles, you would require an action_tile statement for each tile; however, with either of the list style tiles, only a single action_tile statement is required to achieve the same result.
Concerning your error, use the debugging utilities offered by the Visual LISP IDE to ascertain the source and cause of the error. I have written a short tutorial on this here. | 
