ORA-13798: Parameter string cannot be NULL.
Cause : A call to GET_THRESHOLD procedure was made without a required parameter.
Action - How to fix it : DBA Scripts :: www.high-oracle.com/scripts
Specify a valid value for this parameter.
Well, the reason for above error is that i have taken the above script from a 11g database and running it on 10g database. 11g has bring some changes in password management. Below code is executed on 11g and user created successfully, which is expected result.