code: public static This getClassInstanceThis(Object instance, String className) {
          try {
              Object o = getObjectFieldValue(instance, BSHTHIS + className);
              return (This) Primitive.unwrap(o); // unwrap Primitive.Null to null
          } catch (Exception e) {
              throw new InterpreterError("Generated class: Error getting This" + e, e);
          }
      }
docs: Get the instance bsh namespace field from the object instance.
  @return the class instance This object or null if the object has not
  been initialized.