I was getting null pointer exceptions when executing remote server tests from 
the command line:
java.lang.NullPointerException
	at org.apache.jmeter.engine.ConvertListeners.addNode
(ConvertListeners.java:53)
	at org.apache.jorphan.collections.HashTree.traverseInto
(HashTree.java:978)
	at org.apache.jorphan.collections.HashTree.traverse(HashTree.java:962)
	at org.apache.jmeter.engine.ClientJMeterEngine.run
(ClientJMeterEngine.java:120)
	at java.lang.Thread.run(Thread.java:534)

Upon tracing it through I discovered that the same HashTree created on line 453 
of JMeter.java was being accessed by two different ClientJMeterEngine threads. 
I added synchronization around the tree in the run method of the 
ClientJMeterEngine and this seemed to resolve the issues:

	public void run() {
		log.info("running clientengine run method");
		testListeners = new SearchByClass(TestListener.class);
        HashTree tree = getTestTree();
        //synchronized here to resolve NPE issue
        synchronized (tree) {
            tree.traverse(testListeners);
        }
		sampleListeners = new ConvertListeners();

		// TODO this is a temporary fix - see bug 23487
		try {
                    //synchronized here to resolve NPE issue
		    synchronized (tree) {
		        tree.traverse(sampleListeners);
		    }
		} catch (IndexOutOfBoundsException e) {
			log.warn("Error replacing sample listeners", e);
		}