#vulnerable code 
                                public LockNulsTransaction createLockNulsTx(CoinTransferData transferData, String password, String remark) throws Exception {
        LockNulsTransaction tx = new LockNulsTransaction(transferData, password);
        if (StringUtils.isNotBlank(remark)) {
            tx.setRemark(remark.getBytes(NulsContext.DEFAULT_ENCODING));
        }
        tx.setHash(NulsDigestData.calcDigestData(tx.serialize()));
        tx.setSign(getAccountService().signData(tx.getHash(), password));
        return tx;
    } 
                            #location 7 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                public LockNulsTransaction createLockNulsTx(CoinTransferData transferData, String password, String remark) throws Exception {
        LockNulsTransaction tx = new LockNulsTransaction(transferData, password);
        if (StringUtils.isNotBlank(remark)) {
            tx.setRemark(remark.getBytes(NulsContext.DEFAULT_ENCODING));
        }
        tx.setHash(NulsDigestData.calcDigestData(tx.serialize()));
        AccountService accountService = getAccountService();
        if (transferData.getFrom().isEmpty()) {
            throw new NulsRuntimeException(ErrorCode.DATA_ERROR);
        }
        Account account = accountService.getAccount(transferData.getFrom().get(0));
        tx.setSign(accountService.signData(tx.getHash(), account, password));
        return tx;
    }