#vulnerable code 
                             @Override
	public <S extends Serializable> Map<S, Set<Future<Void>>> send(Object message, Collection<S> users) {
		SocketSessionManager manager = SocketSessionManager.getInstance();
		Map<S, Set<Future<Void>>> resultsByUser = new HashMap<>(users.size());

		for (S user : users) {
			Set<String> userChannelIds = getUserChannelIds(user, channel);
			Set<Future<Void>> results = new HashSet<>(userChannelIds.size());

			for (String channelId : userChannelIds) {
				results.addAll(manager.send(channelId, message));
			}

			resultsByUser.put(user, results);
		}

		return resultsByUser;
	} 
                            #location 11 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                             @Override
	public <S extends Serializable> Map<S, Set<Future<Void>>> send(Object message, Collection<S> users) {
		Map<S, Set<Future<Void>>> resultsByUser = new HashMap<>(users.size());

		for (S user : users) {
			Set<String> userChannelIds = userManager.getUserChannelIds(user, channel);
			Set<Future<Void>> results = new HashSet<>(userChannelIds.size());

			for (String channelId : userChannelIds) {
				results.addAll(sessionManager.send(channelId, message));
			}

			resultsByUser.put(user, results);
		}

		return resultsByUser;
	}