#vulnerable code 
                                @Override
    public AccessKey merge(AccessKey key) {
        try {
            long userId = key.getUserId();
            User user = key.getUser();
            Location location = new Location(USER_AND_LABEL_ACCESS_KEY_NS, String.valueOf(userId) + "n" + key.getLabel());
            if (key.getId() == null) {
                key.setId(getId(COUNTERS_LOCATION));
                StoreValue storeValue = new StoreValue.Builder(key.getId()).withLocation(location).build();
                client.execute(storeValue);
            } else {
                AccessKey existing = find(key.getId());
                if (existing.getLabel().equals(key.getLabel())) {
                    DeleteValue delete = new DeleteValue.Builder(location).build();
                    client.execute(delete);
                    StoreValue storeValue = new StoreValue.Builder(key.getId()).withLocation(location).build();
                    client.execute(storeValue);
                }
            }
            Location accessKeyLocation = new Location(ACCESS_KEY_NS, String.valueOf(key.getId()));
            removeReferences(key);
            StoreValue storeOp = new StoreValue.Builder(key).withLocation(accessKeyLocation).build();
            client.execute(storeOp);
            return restoreReferences(key, user);
        } catch (ExecutionException | InterruptedException e) {
            throw new HivePersistenceLayerException("Cannot store access key.", e);
        }
    } 
                            #location 13 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                @Override
    public AccessKey merge(AccessKey key) {
        try {
            long userId = key.getUserId();
            User user = key.getUser();
            Location location = new Location(USER_AND_LABEL_ACCESS_KEY_NS, String.valueOf(userId) + "n" + key.getLabel());
            if (key.getId() == null) {
                key.setId(getId(COUNTERS_LOCATION));
                StoreValue storeValue = new StoreValue.Builder(key.getId()).withLocation(location).build();
                client.execute(storeValue);
            } else {
                AccessKey existing = find(key.getId());
                if (existing != null && existing.getLabel().equals(key.getLabel())) {
                    DeleteValue delete = new DeleteValue.Builder(location).build();
                    client.execute(delete);
                }
                StoreValue storeValue = new StoreValue.Builder(key.getId()).withLocation(location).build();
                client.execute(storeValue);

            }
            Location accessKeyLocation = new Location(ACCESS_KEY_NS, String.valueOf(key.getId()));
            removeReferences(key);
            StoreValue storeOp = new StoreValue.Builder(key).withLocation(accessKeyLocation).build();
            client.execute(storeOp);
            return restoreReferences(key, user);
        } catch (ExecutionException | InterruptedException e) {
            throw new HivePersistenceLayerException("Cannot store access key.", e);
        }
    }