#vulnerable code 
                              private Set<RaftPeerId> getHigherPriorityPeers(RaftConfiguration conf) {
    Set<RaftPeerId> higherPriorityPeers = new HashSet<>();

    int currPriority = conf.getPeer(server.getId()).getPriority();
    Collection<RaftPeer> peers = conf.getPeers();

    for (RaftPeer peer : peers) {
      if (peer.getPriority() > currPriority) {
        higherPriorityPeers.add(peer.getId());
      }
    }

    return higherPriorityPeers;
  } 
                            #location 4 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                              private Set<RaftPeerId> getHigherPriorityPeers(RaftConfiguration conf) {
    Set<RaftPeerId> higherPriorityPeers = new HashSet<>();

    int currPriority = conf.getPeer(server.getId()).getPriority();
    final Collection<RaftPeer> peers = conf.getAllPeers();

    for (RaftPeer peer : peers) {
      if (peer.getPriority() > currPriority) {
        higherPriorityPeers.add(peer.getId());
      }
    }

    return higherPriorityPeers;
  }