Oracle Forms

I have an alert, the following code is attached to the button trigger when-button-pressed:

begin
if :customer.cust_name is null or
:customer.cust_add is null or
:customer.cust_tel is null or
:customer.cust_town is null or
:customer.cust_pcode is null then
show_mandatory_fields_alert;
else
show_save_alert;
commit_form;
end if;
end;

I have a Program Unit procedure of:

PROCEDURE show_mandatory_fields_alert IS
l_button number;
BEGIN
l_button := show_alert('1 or more mandatory fields are empty, please correct');
raise form_trigger_failure;
END;

I know the alert exists because I forgot to put the "raise form_trigger_failure" statement in the procedure and when I compiled the code for the trigger I kept getting a message about not declaring the variable show mandatory_fields_alert.

What I am trying to do is display an alert to the user if certain mandatory fields are not populated. Is there a better way of doing this ?? I don't know if the alert code will work even with the way I have that list above using the OR operator.

Thanks for any help in advance.

Kevin