Set custom UOM while creating a new Part with using code.

I have custom UOM in Windchill loaded to QuantityUnitRB, but I am not able to create a new Part with custom UOM using code.

getting the values via

QuantityUnit[] qu = QuantityUnit.getQuantityUnitSet();

but when I set to part, getting null pointer .

Has anyone come across this. Please suggest the way to do this.

Thank You,