code: @Override
      public ValidationResult validate(PMContext ctx) {
          final ValidationResult res = new ValidationResult();
          final Object fieldvalue = ctx.getFieldValue();
          res.setSuccessful(fieldvalue != null);
          if (!res.isSuccessful()) {
              res.getMessages().add(MessageFactory.error(
                      ctx.getEntity(), ctx.getField(),
                      get("msg", "pm.validator.not.null"),
                      ctx.getField().getId()));
          }
          return res;
      }
docs: The validation method