#vulnerable code 
                                private List<Challenge> fetchChallenges(JSON json) {
        JSON.Array jsonChallenges = json.get("challenges").asArray();
        List<Challenge> cr = new ArrayList<>();
        for (JSON.Value c : jsonChallenges) {
            Challenge ch = getSession().createChallenge(c.asObject());
            if (ch != null) {
                cr.add(ch);
            }
        }
        return cr;
    } 
                            #location 4 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                private List<Challenge> fetchChallenges(JSON json) {
        Session session = getSession();

        return Collections.unmodifiableList(json.get("challenges").asArray().stream()
                .map(JSON.Value::asObject)
                .map(session::createChallenge)
                .collect(toList()));
    }