docs: public List<CmsJspNavElement> getSubNavigation() {
  
          if (m_subNavigation == null) {
              if (m_resource.isFile()) {
                  m_subNavigation = Collections.emptyList();
              } else if (m_navContext == null) {
                  try {
                      throw new Exception("Can not get subnavigation because navigation context is not set.");
                  } catch (Exception e) {
                      LOG.warn(e.getLocalizedMessage(), e);
                      m_subNavigation = Collections.emptyList();
                  }
              } else {
                  CmsJspNavBuilder navBuilder = m_navContext.getNavBuilder();
                  m_subNavigation = navBuilder.getNavigationForFolder(
                      navBuilder.getCmsObject().getSitePath(m_resource),
                      m_navContext.getVisibility(),
                      m_navContext.getFilter());
              }
  
          }
          return m_subNavigation;
  
      }