If a we check try to compare a value to the value of a lua var, but this var is
not definied anywhere yet (nil), this gives an error in the dialog.

If the var is definied in some other dialog, the validator may not see this
dialog bug since it checks all dialogs in one rush and not seperately, and the
dialogs are all checked in anti-alphabetical order.

One problem is that the lua code is fairly non-linear, with every node basically
acting like a goto statement.

The safest way would be to check every node independent of every other node, but
this would mean most nodes would be tested with null variables...