#fixed code
                                public synchronized void syncBlock() {
        this.status = MaintenanceStatus.DOWNLOADING;
        while (true) {
            BestCorrectBlock bestCorrectBlock = checkLocalBestCorrentBlock();
            boolean doit = false;
            long startHeight = 1;


            do {
                if (null == bestCorrectBlock.getLocalBestBlock() && bestCorrectBlock.getNetBestBlockInfo() == null) {
                    doit = true;
                    BlockInfo blockInfo = BEST_HEIGHT_FROM_NET.request(-1);
                    bestCorrectBlock.setNetBestBlockInfo(blockInfo);
                    break;
                }
                startHeight = bestCorrectBlock.getLocalBestBlock().getHeader().getHeight() + 1;
                long interval = TimeService.currentTimeMillis() - bestCorrectBlock.getLocalBestBlock().getHeader().getTime();
                if (interval < (PocConsensusConstant.BLOCK_TIME_INTERVAL_SECOND * 2000)) {
                    doit = false;
                    break;
                }
                if (null == bestCorrectBlock.getNetBestBlockInfo()) {
                    bestCorrectBlock.setNetBestBlockInfo(BEST_HEIGHT_FROM_NET.request(0));
                }
                if (null == bestCorrectBlock.getNetBestBlockInfo()) {
                    break;
                }
                if (bestCorrectBlock.getNetBestBlockInfo().getBestHeight() > bestCorrectBlock.getLocalBestBlock().getHeader().getHeight()) {
                    doit = true;
                    break;
                }
            } while (false);


            if (null == bestCorrectBlock.getNetBestBlockInfo()) {
                return;
            }
            if (doit) {
                downloadBlocks(bestCorrectBlock.getNetBestBlockInfo().getNodeIdList(), startHeight, bestCorrectBlock.getNetBestBlockInfo().getBestHeight());
            } else {
                break;
            }
            long start = TimeService.currentTimeMillis();
            //todo
            long timeout = (bestCorrectBlock.getNetBestBlockInfo().getBestHeight()-startHeight+1)*100;
            while (NulsContext.getInstance().getBestHeight()<bestCorrectBlock.getNetBestBlockInfo().getBestHeight()){
                if(TimeService.currentTimeMillis()>(timeout+start)){
                    break;
                }
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    Log.error(e);
                }
            }
        }
    }
#vulnerable code 
                                public synchronized void syncBlock() {
        this.status = MaintenanceStatus.DOWNLOADING;
        BestCorrectBlock bestCorrectBlock = checkLocalBestCorrentBlock();
        boolean doit = false;
        long startHeight = 1;
        do {
            if (null == bestCorrectBlock.getLocalBestBlock() && bestCorrectBlock.getNetBestBlockInfo() == null) {
                doit = true;
                BlockInfo blockInfo = BEST_HEIGHT_FROM_NET.request(-1);
                bestCorrectBlock.setNetBestBlockInfo(blockInfo);
                break;
            }
            startHeight = bestCorrectBlock.getLocalBestBlock().getHeader().getHeight() + 1;
            long interval = TimeService.currentTimeMillis() - bestCorrectBlock.getLocalBestBlock().getHeader().getTime();
            if (interval < (PocConsensusConstant.BLOCK_TIME_INTERVAL_SECOND * 2000)) {
                doit = false;
                break;
            }
            if (null == bestCorrectBlock.getNetBestBlockInfo()) {
                bestCorrectBlock.setNetBestBlockInfo(BEST_HEIGHT_FROM_NET.request(0));
            }
            if (null == bestCorrectBlock.getNetBestBlockInfo()) {
                break;
            }
            if (bestCorrectBlock.getNetBestBlockInfo().getBestHeight() > bestCorrectBlock.getLocalBestBlock().getHeader().getHeight()) {
                doit = true;
                break;
            }
        } while (false);
        if (null == bestCorrectBlock.getNetBestBlockInfo()) {
            return;
        }
        if (doit) {
            downloadBlocks(bestCorrectBlock.getNetBestBlockInfo().getNodeIdList(), startHeight, bestCorrectBlock.getNetBestBlockInfo().getBestHeight());
        } else {
            this.status = MaintenanceStatus.SUCCESS;
        }
    } 
                            #location 13 
                            #vulnerability type NULL_DEREFERENCE