#vulnerable code 
                              @Override
  public Ring addRing(int ringNum) throws IOException {
    try {
      Ring rc = ZkRing.create(zk, ringGroupPath, ringNum, this, isUpdating() ? getUpdatingToVersion() : getCurrentVersion());
      ringsByNumber.put(rc.getRingNumber(), rc);
      return rc;
    } catch (Exception e) {
      throw new IOException(e);
    }
  } 
                            #location 4 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                              @Override
  public Ring addRing(int ringNum) throws IOException {
    try {
      Ring rc = ZkRing.create(zk, ringGroupPath, ringNum, this,
        isUpdating() ? getUpdatingToVersion() : getCurrentVersion());
      ringsByNumber.put(rc.getRingNumber(), rc);
      return rc;
    } catch (Exception e) {
      throw new IOException(e);
    }
  }