Trigger implementation in SYS as SYSDBA issue

Question

I am trying to implement a trigger and getting following errors:
CREATE OR REPLACE TRIGGER gsrvr_trace
ERROR at line 1:
ORA-00604: error occurred at recursive SQL level 1
ORA-01400: cannot insert NULL into ("SYS"."OBJ$"."NAME")
I use Oracle 9.2.0.8 on Linux.
Creation script:
CREATE OR REPLACE TRIGGER gsrvr_trace
AFTER LOGON ON DATABASE
DECLARE
isid PLS_INTEGER;
iserial PLS_INTEGER;
iaudsid PLS_INTEGER;
sprogram v$session.program%TYPE;
BEGIN
SELECT SYS_CONTEXT ('USERENV', 'SESSIONID')
INTO iaudsid
FROM DUAL;
SELECT SID, serial#, program
INTO isid, iserial, sprogram
FROM v$session
WHERE audsid = iaudsid;
DBMS_OUTPUT.put_line (sprogram);
IF (LENGTH (sprogram) > 0)
THEN
IF ( INSTR (UPPER (sprogram), 'GSRVR') > 0
OR INSTR (UPPER (sprogram), 'ARCMAP') > 0
THEN
EXECUTE IMMEDIATE 'alter session set tracefile_identifier="_'
|| SUBSTR
(sprogram,
1,
CASE
WHEN INSTR (sprogram, '@') = 0
THEN LENGTH (sprogram)
ELSE INSTR (sprogram, '@') - 1
END
|| '_'
|| USER
|| "";
EXECUTE IMMEDIATE 'alter session set timed_statistics=true';
EXECUTE IMMEDIATE 'alter set max_dump_size=2147483647';
EXECUTE IMMEDIATE 'alter session set events "10046 trace name context forever,level 12"';
EXECUTE IMMEDIATE 'alter session set statistics_level=ALL';
END IF;
END IF;
END;
/

Answer 1

The problem was only in quotes!!!
No " (double quotes), only ' (single ones!):)