code: @Override
      public void validate(ValidationHelper helper, Context context, String key, XML t) {
          if (t != null) {
              if (StringUtils.isNotBlank(t.getNamespace())) {
                  String namespace = t.getNamespace();
                  if (!ValidatorUtils.isValidURI(namespace)) {
                      final String message = Tr.formatMessage(tc, "invalidUri", namespace);
                      helper.addValidationEvent(new ValidationEvent(ValidationEvent.Severity.ERROR, context.getLocation(), message));
                  }
              }
          }
      }
docs: {@inheritDoc}