#vulnerable code 
                              @Override
  public List<String> listRecursive(final String path) throws KeeperException {
    assertClusterIdFlagTrue();

    // namespace the path since we're using zookeeper directly
    final String namespace = emptyToNull(client.getNamespace());
    final String namespacedPath = ZKPaths.fixForNamespace(namespace, path);

    try {
      final List<String> paths = ZKUtil.listSubTreeBFS(
          client.getZookeeperClient().getZooKeeper(), namespacedPath);

      if (isNullOrEmpty(namespace)) {
        return paths;
      } else {
        // hide the namespace in the paths returned from zookeeper
        final ImmutableList.Builder<String> builder = ImmutableList.builder();
        for (final String p : paths) {
          final String fixed;
          if (p.startsWith("/" + namespace)) {
            fixed = (p.length() > namespace.length() + 1)
                    ? p.substring(namespace.length() + 1)
                    : "/";
          } else {
            fixed = p;
          }

          builder.add(fixed);
        }

        return builder.build();
      }
    } catch (Exception e) {
      propagateIfInstanceOf(e, KeeperException.class);
      throw propagate(e);
    }
  } 
                            #location 21 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                              @Override
  public List<String> listRecursive(final String path) throws KeeperException {
    assertClusterIdFlagTrue();
    try {
      return ZKUtil.listSubTreeBFS(client.getZookeeperClient().getZooKeeper(), path);
    } catch (Exception e) {
      propagateIfInstanceOf(e, KeeperException.class);
      throw propagate(e);
    }
  }