Search notes:

Oracle: A NULL instance of an XMLTYPE cannot be created

select
   xmltype(null)
from
   dual;

The following PL/SQL block throws ORA-06550: line 4, column 7: PLS-00307: too many declarations of 'XMLTYPE' match this call

declare
   x xmltype;
begin
   x:=xmltype(null);
end;
/

Trying to specify a data type for the null value results in ORA-19032: expected XML tag not present:

select
   xmltype(cast (null as varchar2(1)))
from
   dual;

See also