Send message to particular client via messaging

I want to send message to particular user via messaging (consumer/producer).

I have following code for this:

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public void sendMessageToClient(Object msgBody, String clientId) {
        MessageBroker.getMessageBroker(null).routeMessageToMessageClient(createMessage(msgBody), getMessageClient(clientId));
    }

    private MessageClient getMessageClient(String clientId) {
        MessageBroker messageBroker = MessageBroker.getMessageBroker(null);
        Object serviceId = messageBroker.getDestinationService().get("informer");
        MessageService messageService = (MessageService) messageBroker.getServices().get(serviceId);
        MessageDestination messageDestination = (MessageDestination) messageService.getDestination("informer");
        MessageClient messageClient = messageDestination.getSubscriptionManager().getSubscriber(clientId);
        return messageClient;
    }

    private AsyncMessage createMessage(Object msgBody) {
        AsyncMessage msg = new AsyncMessage();
        // msg.setClientId(UUIDUtils.createUUID(true));
        msg.setTimestamp(System.currentTimeMillis());
        msg.setMessageId(UUIDUtils.createUUID(true));
        msg.setDestination("informer");
        msg.setBody(msgBody);
        // msg.setHeader("sender", "From the server");
        return msg;
    }

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and corresponding configurations in messaging-config.xml :

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<destination id="informer">
        <properties>
            <network>
                <session-timeout>0</session-timeout>
            </network>
            <server>
                <max-cache-size>1000</max-cache-size>
                <message-time-to-live>0</message-time-to-live>
                <durable>false</durable>
            </server>
        </properties>
        <channels>
            <channel ref="channel-rtmp" />
            <channel ref="channel-amf" />
            <channel ref="channel-polling-amf" />
        </channels>
    </destination>

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but it throws NullPointerException at flex.messaging.MessageBroker.routeMessageToMessageClient(MessageBroker.java:1372)

any suggestions?