oracle open form with parameter

hi
i have form open within a button of another form passing a value to open as the follwoing :\
1-st form

 COPY(null,'GLOBAL.form_id');
  :global.form_id := rec_id;
   CALL_FORM('PO');

Select allOpen in new window

PO form

If :global.form_id is not null then
v_ID_where := 'po_id = ' || :global.form_id;
Set_block_property('PO_M', default_where, v_ID_where);
go_block('PO_M');
Execute_Query;
ERASE('GLOBAL.form_id');

Select allOpen in new window

the form called ok when it open from the 1st form , but when i open po form alone , it sent error message that variable not found