Uncaught Ext.Error: You're trying to decode an invalid JSON String: Form Submission using Extjs and Spring mvc
===

I am getting following error which submitting my extjs form. 

***Uncaught Ext.Error: You're trying to decode an invalid JSON String***

Please find my code below. Below is my js file. 

    Ext.onReady(function() {

			var simple = Ext.create('Ext.form.Panel', {
						
						frame : true,
						title : 'Login Form',
						bodyStyle : 'padding:5px 5px 0',
						width : 350,
						fieldDefaults : {
							msgTarget : 'side',
							labelWidth : 75
						},
						defaultType : 'textfield',
						defaults : {
							anchor : '100%'
						},

						items : [{
									fieldLabel : 'User Name',
									name : 'userName',
									allowBlank : false,
									emptyText : 'UserName'
								}, {
									fieldLabel : 'Password',
									name : 'password',
									allowBlank : false,
									inputType : 'password',
									emptyText : 'Password'
								}],

						buttons : [{
							text : 'Save',
							handler : function() {
								var form = this.up('form').getForm();
								form.submit({
											url : saveFormUrl
										//	waitMsg : 'Sending the info...',
										//	success : function(fp, o) {
										//		Ext.Msg.alert('Success',
										//				'Form submitted.');
										//	}
										});
							}
						}, {
							text : 'Cancel'
						}]
					});
			simple.render(document.body);
			simple.getEl().center();
		});


This is my controller class..

    @Controller
    public class UserController {

	private static final Logger logger = LoggerFactory
			.getLogger(TController.class);

	private TService tService = null;

	@Autowired
	public void setTService(TService tService) {
		this.tService = tService;
	}

	@RequestMapping(value = "/index.html", method = RequestMethod.GET)
	public String home() {
		System.out.println("Welcome home!");
		return "login";
	}
	
	@RequestMapping(value = "/save-form.html", method = RequestMethod.POST)
	public ModelAndView submitData(User user){
		System.out.println("User name:"+user.getUserName());
		ModelAndView mv = new ModelAndView("htmlLinks");
		return mv;
	}


This is my save-form.html file.. 


    <%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>
    <%@ page session="false"%>
    <c:set var="ctx" value="${pageContext.request.contextPath}" />
    <html>
    <head>
    <title>POC</title>

    </head>
    <body>
     Welcome User !!
    </body>
    </html>


Could anyone tell me what am I doing wrong? and what should I do as a solution? I am using ExtJs 4 and Spring MVC. 


Thanks. 