Hibernate Configuration.addClass(tableClass) and Session.createCriteria(tableClass)

I got run time error throw to NullPointerException
when I do
Configuration config = new Configuration();
config.addClass(myClassA.class).addClass(myClassB.class);

Why?

But I don't get any run time error when

do

SesstionFactory sf = config.buildSessionFactory();

Session session = sf.openSession();
List list = 
session.createCriteria(myClassA).createCriteria(myClassB).list();

Instead session.createCriteria, I would like to try

List list = session.createQuery("....").list();

But I get run time error in config.addClass().

How to resolve this problem?

Thank you in advance!