if :block.item1 is null or :block.item4 is null or :block.itemx is null then
   message('Incomplete data');
   raise form_trigger_failure;
else
   commit;
end if;

if ...
   message
   raise
end if

if :block.item1 is null or :block.item4 is null or :block.itemx is null ...

declare
  t_item  varchar2(64);
  t_mandatory boolean;
  t_string varchar2(256);
  t_display_message varchar2(1);
  t_block varchar2(64);
begin
  t_item    := get_block_property(t_block,first_item);
--loop through all items in the current block of a form
  while t_item is not null
  loop
  	  if get_item_property(t_block||'.'||t_item, item_type)
  	  in ('TEXT ITEM','LIST')
      then 
            t_manadatory := get_item_property(t_block||'.'||t_item, required);
      else t_manadatory := 'FALSE';
      end if;
      if t_manadatory = 'TRUE'
      then
            if name_in(t_block||'.'||t_item) is null
            then
                  t_display_message := 'J';
                  t_string := t_string ||' '||get_item_property(t_block||'.'||t_item , prompt_text);
            end if;
      end if;
      t_item      := get_item_property(t_block||'.'||t_item, nextitem);
  end loop;
  if t_display_message = 'J'
  then
        message('Please fill these:'||t_string );
        message('Please fill these:'||t_string );
      
  else
    --all the mandatory fields have been filled, do something else. 
  end if;

end;

if :block.item1 is null or :block.item4 is null or :block.itemx is null ...