code: public String getValueString(String label) {
          PrimitiveObject o = getPrimitiveObject(VALUE, label, ObjectUtil.STRING, "String");
          if (o == null) {
              return null;
          }
          return (String) o.getObject();
      }
docs: Get value {@link String} value
  @param label target label
  @return {@link String} value of the label. If it is not null.