#vulnerable code 
                              private void setupServer(){
    servers = new ArrayList<>(peers.size());
    singleDataStreamStateMachines = new ArrayList<>(peers.size());
    // start stream servers on raft peers.
    for (int i = 0; i < peers.size(); i++) {
      SingleDataStreamStateMachine singleDataStreamStateMachine = new SingleDataStreamStateMachine();
      singleDataStreamStateMachines.add(singleDataStreamStateMachine);
      DataStreamServerImpl streamServer;
      if (i == 0) {
        // only the first server routes requests to peers.
        List<RaftPeer> otherPeers = new ArrayList<>(peers);
        otherPeers.remove(peers.get(i));
        streamServer = new DataStreamServerImpl(
            peers.get(i), properties, null, singleDataStreamStateMachine, otherPeers);
      } else {
        streamServer = new DataStreamServerImpl(
            peers.get(i), singleDataStreamStateMachine, properties, null);
      }
      servers.add(streamServer);
      streamServer.getServerRpc().startServer();
    }

    // start peer clients on stream servers
    for (DataStreamServerImpl streamServer : servers) {
      ((NettyServerStreamRpc) streamServer.getServerRpc()).startClientToPeers();
    }
  } 
                            #location 20 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                              private void setupServer(){
    servers = new ArrayList<>(peers.size());
    singleDataStreamStateMachines = new ArrayList<>(peers.size());
    // start stream servers on raft peers.
    for (int i = 0; i < peers.size(); i++) {
      SingleDataStreamStateMachine singleDataStreamStateMachine = new SingleDataStreamStateMachine();
      singleDataStreamStateMachines.add(singleDataStreamStateMachine);
      final DataStreamServerImpl streamServer = new DataStreamServerImpl(
          peers.get(i), singleDataStreamStateMachine, properties, null);
      final DataStreamServerRpc rpc = streamServer.getServerRpc();
      if (i == 0) {
        // only the first server routes requests to peers.
        List<RaftPeer> otherPeers = new ArrayList<>(peers);
        otherPeers.remove(peers.get(i));
        rpc.addPeers(otherPeers);
      }
      rpc.start();
      servers.add(streamServer);
    }
  }