Issue #3271397 by RenatoG: Create a validation to full stop in the beginning...

      $form_state->setErrorByName('class][third_party_settings][block_class][classes', $this->t("In class is allowed only letters, numbers, hyphen and underline"));

      return FALSE;

    }

    $classes = explode(' ', $classes);

    foreach ($classes as $class) {

      // Validate if the first char is a point and say that isn't necessary.

      if (strpos($class, '.') === 0) {

        $form_state->setErrorByName('class][third_party_settings][block_class][classes', $this->t("Isn't necessary add point in the beginning of class"));

        return FALSE;

      }

    }

  }