smartGWT ListGrid JSON issue-Collection of common programming errors

I have created a REST API at URL: http://localhost:8888/rest/contacts with this JSON output:

{
    "contact": {
        "address": [
            {
                "city":"Shanghai",
                "street":"Long Hua Street"
            },
            {
                "city":"Shanghai",
                "street":"Dong Quan Street"
            }
        ],
        "id": "huangyim",
        "name": "Huang Yi Ming"
   }
}

I want to print only the id value in a smartGWT ListGrid.

public class ExampleEntry implements EntryPoint {
    #Override
    public void onModuleLoad() {
        DataSource dataSource = new DataSource();  
        dataSource.setDataFormat(DSDataFormat.JSON);  
        dataSource.setDataURL("http://localhost:8888/rest/contacts");
        dataSource.setRecordXPath("/contact");

        DataSourceTextField field = new DataSourceTextField("id", "id");
        dataSource.addField(field);        

        final ListGrid grid = new ListGrid();  
        grid.setDataSource(dataSource);
        grid.setAutoFetchData(true);  
        grid.draw();
    }
}

But it is throwing the following exception:

15:33:12.766 [ERROR] [jerseyexample] 15:33:12.747:XRP2:WARN:RPCManager:xmlHttpRequest.getAllResponseHeaders() returned null
com.smartgwt.client.core.JsObject$SGWT_WARN: 15:33:12.747:XRP2:WARN:RPCManager:xmlHttpRequest.getAllResponseHeaders() returned null
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source)
    at java.lang.reflect.Constructor.newInstance(Unknown Source)
    at com.google.gwt.dev.shell.MethodAdaptor.invoke(MethodAdaptor.java:105)
    at com.google.gwt.dev.shell.MethodDispatch.invoke(MethodDispatch.java:71)
    at com.google.gwt.dev.shell.OophmSessionHandler.invoke(OophmSessionHandler.java:172)
    at com.google.gwt.dev.shell.BrowserChannelServer.reactToMessages(BrowserChannelServer.java:293)
    at com.google.gwt.dev.shell.BrowserChannelServer.processConnection(BrowserChannelServer.java:547)
    at com.google.gwt.dev.shell.BrowserChannelServer.run(BrowserChannelServer.java:364)
    at java.lang.Thread.run(Unknown Source)

I tried searching google to find a fix but that did not help. Please let me know if anyone knows a solution for this issue.