output: #fixed code
                                private void validatePieceAsync(final SharedTorrent torrent, final Piece piece, String torrentHash, SharingPeer peer) {
      try {
        synchronized (piece) {
  
          if (piece.isValid()) return;
  
          piece.validate(torrent, piece);
          if (piece.isValid()) {
            piece.finish();
            // Send a HAVE message to all connected peers, which don't have the piece
            PeerMessage have = PeerMessage.HaveMessage.craft(piece.getIndex());
            for (SharingPeer remote : getConnectedPeers()) {
              if (remote.getTorrent().getHexInfoHash().equals(torrentHash) &&
                      !remote.getAvailablePieces().get(piece.getIndex()))
                remote.send(have);
            }
  
            final boolean isTorrentComplete;
            synchronized (torrent) {
              torrent.removeValidationFuture(piece);
              torrent.notifyPieceDownloaded(piece, peer);
  
              boolean isCurrentPeerSeeder = peer.getAvailablePieces().cardinality() == torrent.getPieceCount();
              //if it's seeder we will send not interested message when we download full file
              if (!isCurrentPeerSeeder) {
                if (torrent.isAllPiecesOfPeerCompletedAndValidated(peer)) {
                  peer.notInteresting();
                }
              }
  
              isTorrentComplete = torrent.isComplete();
  
              if (isTorrentComplete) {
                logger.debug("Download of {} complete.", torrent.getDirectoryName());
  
                torrent.finish();
              }
            }
  
            if (isTorrentComplete) {
  
              LoadedTorrent announceableTorrent = torrentsStorage.getLoadedTorrent(torrentHash);
  
              if (announceableTorrent == null) return;
  
              AnnounceableInformation announceableInformation = announceableTorrent.createAnnounceableInformation();
  
  
              try {
                announce.getCurrentTrackerClient(announceableInformation)
                        .announceAllInterfaces(COMPLETED, true, announceableInformation);
              } catch (AnnounceException e) {
                logger.debug("unable to announce torrent {} on tracker {}", torrent, torrent.getAnnounce());
              }
  
              for (SharingPeer remote : getPeersForTorrent(torrentHash)) {
                remote.notInteresting();
              }
  
            }
          } else {
            torrent.markUncompleted(piece);
            logger.info("Downloaded piece #{} from {} was not valid ;-(. Trying another peer", piece.getIndex(), peer);
            peer.getPoorlyAvailablePieces().set(piece.getIndex());
          }
        }
      } catch (Throwable e) {
        torrent.markUncompleted(piece);
        logger.warn("unhandled exception in piece {} validation task", e);
      }
      torrent.handlePeerReady(peer);
    }
input: #vulnerable code 
                                private void validatePieceAsync(final SharedTorrent torrent, final Piece piece, String torrentHash, SharingPeer peer) {
      try {
        synchronized (piece) {
  
          if (piece.isValid()) return;
  
          piece.validate(torrent, piece);
          if (piece.isValid()) {
            piece.finish();
            // Send a HAVE message to all connected peers, which don't have the piece
            PeerMessage have = PeerMessage.HaveMessage.craft(piece.getIndex());
            for (SharingPeer remote : getConnectedPeers()) {
              if (remote.getTorrent().getHexInfoHash().equals(torrentHash) &&
                      !remote.getAvailablePieces().get(piece.getIndex()))
                remote.send(have);
            }
  
            final boolean isTorrentComplete;
            synchronized (torrent) {
              torrent.removeValidationFuture(piece);
              torrent.notifyPieceDownloaded(piece, peer);
  
              boolean isCurrentPeerSeeder = peer.getAvailablePieces().cardinality() == torrent.getPieceCount();
              //if it's seeder we will send not interested message when we download full file
              if (!isCurrentPeerSeeder) {
                if (torrent.isAllPiecesOfPeerCompletedAndValidated(peer)) {
                  peer.notInteresting();
                }
              }
  
              isTorrentComplete = torrent.isComplete();
  
              if (isTorrentComplete) {
                logger.debug("Download of {} complete.", torrent.getDirectoryName());
  
                torrent.finish();
              }
            }
  
            if (isTorrentComplete) {
  
              LoadedTorrent announceableTorrent = torrentsStorage.getLoadedTorrent(torrentHash);
  
              if (announceableTorrent == null) return;
  
              AnnounceableInformation announceableInformation = announceableTorrent.createAnnounceableInformation();
  
  
              try {
                announce.getCurrentTrackerClient(announceableInformation)
                        .announceAllInterfaces(COMPLETED, true, announceableInformation);
              } catch (AnnounceException e) {
                logger.debug("unable to announce torrent {} on tracker {}", torrent, torrent.getAnnounce());
              }
  
              for (SharingPeer remote : getPeersForTorrent(torrentHash)) {
                remote.notInteresting();
              }
  
            }
          } else {
            torrent.markUncompleted(piece);
            logger.info("Downloaded piece #{} from {} was not valid ;-(. Trying another peer", piece.getIndex(), peer);
            peer.getPoorlyAvailablePieces().set(piece.getIndex());
          }
        }
      } catch (Throwable e) {
        torrent.markUncompleted(piece);
        logger.warn("unhandled exception in piece {} validation task", e);
      }
    } 
                              #location 51 
                              #vulnerability type NULL_DEREFERENCE