package variables

I have an object:

CREATE OR REPLACE TYPE o_societa UNDER object_arc (

        DESCR VARCHAR2(50),
        CONSTRUCTOR FUNCTION o_societa RETURN SELF AS RESULT,
        OVERRIDING MEMBER PROCEDURE seleziona,
        OVERRIDING MEMBER PROCEDURE stampa_elenco

In a PLSQL block I declare:

CREATE OR REPLACE PACKAGE BODY apkg_arc AS

        oggetto_arc object_arc;         

        PROCEDURE seleziona (arc VARCHAR2, funz VARCHAR2) AS
	BEGIN
                IF arc = 'societa' THEN
                        oggetto_arc := o_societa;
                END IF;



		-- write a web form with PROCEDURE stampa_elenco

		-- as target

                oggetto_arc.seleziona;
		....
	END;

        PROCEDURE stampa_elenco (name_array owa.vc_arr,
                value_array owa.vc_arr) AS
        BEGIN
                -- oggetto_arc := o_societa;
                htp.header(1,'ID => '||oggetto_arc.id);

                oggetto_arc.stampa_elenco;
		...
	END;

but in 'stampa_elenco' i got

ORA-30625: method dispatch on NULL SELF argument is disallowed

if I renitialize oggetto_arc in 'stampa_elenco', its work but ... the global var 'oggetto_arc' does not mantain the initialization made in 'seleziona' ??

tanks in advance ...

-- 
#include <stdio.h>
int main(void){char c[]={10,65,110,116,111,110,105,111,32,98,97,114,98,111,110,
101,32,60,104,105,110,100,101,109,105,116,64,116,105,115,99,97,108,105,110,101,
116,46,105,116,62,10,10,0};printf("%s",c);return 0;}