#vulnerable code 
                              @Override
  public void updateComplete() throws IOException {
    try {
      if (zk.exists(ringPath + CURRENT_VERSION_PATH_SEGMENT, false) != null) {
        zk.setData(ringPath + CURRENT_VERSION_PATH_SEGMENT, getUpdatingToVersionNumber().toString().getBytes(), -1);
      } else {
        zk.create(ringPath + CURRENT_VERSION_PATH_SEGMENT, getUpdatingToVersionNumber().toString().getBytes(), Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
      }
      zk.delete(ringPath + UPDATING_TO_VERSION_PATH_SEGMENT, -1);
    } catch (Exception e) {
      throw new IOException(e);
    }
  } 
                            #location 7 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                              @Override
  public void updateComplete() throws IOException {
    try {
      currentVersionNumber.set(getUpdatingToVersionNumber());
      updatingToVersionNumber.set(null);
    } catch (Exception e) {
      throw new IOException(e);
    }
  }