异常提示： Exception in thread "Thread-0" java.lang.NullPointerException ？

package flex.testdrive.feed;

import java.util.*;
import flex.messaging.MessageBroker;
import flex.messaging.messages.AsyncMessage;
import flex.messaging.util.UUIDUtils;

public class Feed {
    private static FeedThread thread;

    public Feed() {
    }
    public static void main(String args[]){
        Feed fd = new Feed();
        fd.start();
        
        
    }
    public void start() {
        if (thread == null) {
            thread = new FeedThread();
            thread.start();
        }
    }

    public void stop() {
        thread.running = false;
        thread = null;
    }

    public static class FeedThread extends Thread {

        public boolean running = true;

        public void run() {
            MessageBroker msgBroker = MessageBroker.getMessageBroker(null);
            String clientID = UUIDUtils.createUUID();

            Random random = new Random();
            double initialValue = 35;
            double currentValue = 35;
            double maxChange = initialValue * 0.005;

            while (running) {
                double change = maxChange - random.nextDouble() * maxChange * 2;
                double newValue = currentValue + change;

                if (currentValue < initialValue + initialValue * 0.15
                        && currentValue > initialValue - initialValue * 0.15) {
                    currentValue = newValue;
                } else {
                    currentValue -= change;
                }

                AsyncMessage msg = new AsyncMessage();
                msg.setDestination("feed");
                msg.setClientId(clientID);
                msg.setMessageId(UUIDUtils.createUUID());
                msg.setTimestamp(System.currentTimeMillis());
                msg.setBody(new Double(currentValue));
            [color=#FF0000]    msgBroker.routeMessageToService(msg, null); //这个地方出现错误[/color]   

                System.out.println("" + currentValue);

                try {
                    Thread.sleep(300);
                } catch (InterruptedException e) {
                }

            }
        }
    }

}