A dialog doesn't have area's. A dialog has items, which can be listboxes, buttons, edit boxes, etc. The on DIALOG event responds when you click inside such an item. When you click on the GUI itself, not in an item, nothing gets triggered, and so there are no errors either. When you click on a button to do something but some variable isn't set, then you can use something like:
on *:DIALOG:tablename:id-of-button: {
if (!%varname) echo -a var has no value !!
}
and of course you can dump that echo into another dialog.