Replace "P10_ITEM" with the item name of your select list and make sure it executes on page load.
I don't think it is possible to arrange the order of the null option in a select list.- is close. You can't control the order of the Null value if you add the Null value through the GUI. However, it can be added and controlled as part of a SELECT ...
select d, r from ( select 'YES' d, 'Y' r from dual union all select 'No' d, 'N' r from dual union all select Null d, '<Null>' r from dual ) order by 1 desc NULLS last