code: public void readTreeLevel(CmsObject cms, CmsUUID parentId) {
  
          try {
              CmsResource parent = cms.readResource(parentId, m_filter);
              List<CmsResource> children = cms.readResources(parent, m_filter, false);
  
              // sets the parent to leaf mode, in case no child folders are present
              setChildrenAllowed(parentId, !children.isEmpty());
  
              for (CmsResource resource : children) {
                  addTreeItem(cms, resource, parentId);
              }
          } catch (CmsException e) {
              CmsErrorDialog.showErrorDialog(
                  CmsVaadinUtils.getMessageText(Messages.ERR_EXPLORER_CAN_NOT_READ_RESOURCE_1, parentId),
                  e);
              LOG.error(e.getLocalizedMessage(), e);
          }
      }
docs: Reads the given tree level.<p>
  @param cms the CMS context
  @param parentId the parent id