Form checking empty field and non-digit data

1.Form

2.

declare

    chk_button number;

begin

    if error_type = 'FRM' and error_code = 40202 then

        message ('Enter Suplier''s number.');

        set_alert_property('BUSS_ALERT', title,'Business Rule Violation');

        set_alert_property('BUSS_ALERT', alert_message_text ,'Supplier''s number cannot be left empty.');

        chk_button :=show_alert ('BUSS_ALERT');

    elsif

        error_type = 'FRM' and error_code = 50016 then

        message ('Enter Suplier''s number.');

        set_alert_property('BUSS_ALERT', title,'Business Rule Violation');

        set_alert_property('BUSS_ALERT', alert_message_text, 'Supplier''s number cannot contain non-digits.');

        chk_button :=show_alert ('BUSS_ALERT');

            else

                message (SQLCODE||SQLERRM);

            end if;

 END;

3. Output