Forms :: Verification Of Item Manually?

I want to check an item manually. Here is the

//when-validate-item
declare
i number;
begin
if :a.b is null then
i:=show_alert('null');
else if :a.b between 1900 and 2013 then
message('date valid');
else
i:=show_alert('date_invalid');
:a.b:=null;
raise form_trigger_failure;
end if;
end if;
exception when others then
i:=show_alert('exception');
end;

the problem for example when I entered a value "3000" I received two messages (the first on the alert "date_invalid" and the 2nd on the exception message) but normally I must received one message, the relative "date_invalid"