PL/SQL: Use 'IF' statement outside 'WITH ... AS' clause

I am trying to write a procedure where I use Subquery Factoring 'WITH .. AS', but when I use 'IF .. THEN' before it, I get syntax error, I don't know how should I write it, any help?

  BEGIN
  OPEN my_SYS_REFCURSOR FOR
   IF .. IS NULL
   THEN
     WITH SomeName
          AS (SELECT.....);

You simply need to separate the IF statement from the OPEN:

declare
    my_sys_refcursor sys_refcursor;
begin
    if (1=1) then /* condition satisfied, cursor from some table */
        open my_sys_refcursor for
        with somename as ( select '1' as one from dual)
        select one
        from somename;
    else  /* condition not satisfied, select from different tables */
        open my_sys_refcursor for
        with someOthername as ( select 'one' as one from dual)
        select one
        from someOthername;
    end if;
end;