Okay, I suppose you could do it this way:
ui list box("Site:","www.google,www.bing",#site)
ui list box("Location:",".com,.ca,.co.uk,.mx",#location)
if($comparison(#site,"=",$nothing)) {
then {
alert("Please choose a site first.")
stop script
}
else {
}
}
if($comparison(#location,"=",$nothing)) {
then {
alert("Please choose a location first.")
stop script
}
else {
}
}