#vulnerable code 
                                public boolean containsSpend(String key) {
        for (int i = 0; i < unSpends.size(); i++) {
            UtxoOutput output = unSpends.get(i);
            if (key.equals(output.getTxHash().getDigestHex() + "-" + output.getIndex())) {
                return true;
            }
        }
        return false;
    } 
                            #location 4 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                public boolean containsSpend(String key) {
        for (int i = 0; i < unSpends.size(); i++) {
            UtxoOutput output = unSpends.get(i);
            if (key.equals(output.getKey())) {
                return true;
            }
        }
        return false;
    }