Oracle - Error:'ORA-01400:no se puede insertar NULL en

Prueba esto:

create or replace trigger EDITIONS_COR
  before insert or update on EDITIONS
  for each row
begin
  if INSERTING then
    select EDITIONS_SEQ.nextval into :new.ID from DUAL;
  end if;
  :new.CORDATE:=SYSDATE;
  :new.USERNAME:=USER;
end;