Erro na validação do form

O erro só ocorre ao inserir sem preencher os dados do form, se preencher e salvar não acontece e salva normalmente.

HTTP Status 500 - org.springframework.beans.NotReadablePropertyException: Invalid property 'produto' of bean class [br.com.casadocodigo.loja.models.Produto]: Bean property 'produto' is not readable or has an invalid getter method: Does the return type of the getter match the parameter type of the setter?

type Exception report

message org.springframework.beans.NotReadablePropertyException: Invalid property 'produto' of bean class [br.com.casadocodigo.loja.models.Produto]: Bean property 'produto' is not readable or has an invalid getter method: Does the return type of the getter match the parameter type of the setter?

description The server encountered an internal error that prevented it from fulfilling this request.

exception

org.apache.jasper.JasperException: org.springframework.beans.NotReadablePropertyException: Invalid property 'produto' of bean class [br.com.casadocodigo.loja.models.Produto]: Bean property 'produto' is not readable or has an invalid getter method: Does the return type of the getter match the parameter type of the setter?
    org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:565)
    org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:481)
    org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:385)
    org.apache.jasper.servlet.JspServlet.service(JspServlet.java:329)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
    org.springframework.web.servlet.view.InternalResourceView.renderMergedOutputModel(InternalResourceView.java:172)
    org.springframework.web.servlet.view.AbstractView.render(AbstractView.java:303)
    org.springframework.web.servlet.DispatcherServlet.render(DispatcherServlet.java:1228)
    org.springframework.web.servlet.DispatcherServlet.processDispatchResult(DispatcherServlet.java:1011)
    org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:955)
    org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:877)
    org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:961)
    org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:863)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:648)
    org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:837)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
    org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:88)
    org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
root cause

org.springframework.beans.NotReadablePropertyException: Invalid property 'produto' of bean class [br.com.casadocodigo.loja.models.Produto]: Bean property 'produto' is not readable or has an invalid getter method: Does the return type of the getter match the parameter type of the setter?
    org.springframework.beans.BeanWrapperImpl.getPropertyValue(BeanWrapperImpl.java:705)
    org.springframework.beans.BeanWrapperImpl.getNestedBeanWrapper(BeanWrapperImpl.java:551)
    org.springframework.beans.BeanWrapperImpl.getBeanWrapperForPropertyPath(BeanWrapperImpl.java:528)
    org.springframework.beans.BeanWrapperImpl.getPropertyValue(BeanWrapperImpl.java:694)
    org.springframework.validation.AbstractPropertyBindingResult.getActualFieldValue(AbstractPropertyBindingResult.java:99)
    org.springframework.validation.AbstractBindingResult.getFieldValue(AbstractBindingResult.java:229)
    org.springframework.web.servlet.support.BindStatus.<init>(BindStatus.java:120)
    org.springframework.web.servlet.tags.form.AbstractDataBoundFormElementTag.getBindStatus(AbstractDataBoundFormElementTag.java:168)
    org.springframework.web.servlet.tags.form.ErrorsTag.shouldRender(ErrorsTag.java:141)
    org.springframework.web.servlet.tags.form.AbstractHtmlElementBodyTag.writeTagContent(AbstractHtmlElementBodyTag.java:48)
    org.springframework.web.servlet.tags.form.AbstractFormTag.doStartTagInternal(AbstractFormTag.java:84)
    org.springframework.web.servlet.tags.RequestContextAwareTag.doStartTag(RequestContextAwareTag.java:80)
    org.apache.jsp.WEB_002dINF.views.produtos.form_jsp._jspService(form_jsp.java:176)
    org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
    org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:443)
    org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:385)
    org.apache.jasper.servlet.JspServlet.service(JspServlet.java:329)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
    org.springframework.web.servlet.view.InternalResourceView.renderMergedOutputModel(InternalResourceView.java:172)
    org.springframework.web.servlet.view.AbstractView.render(AbstractView.java:303)
    org.springframework.web.servlet.DispatcherServlet.render(DispatcherServlet.java:1228)
    org.springframework.web.servlet.DispatcherServlet.processDispatchResult(DispatcherServlet.java:1011)
    org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:955)
    org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:877)
    org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:961)
    org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:863)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:648)
    org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:837)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
    org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:88)
    org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)

form.jsp

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>
<%@ taglib uri="http://www.springframework.org/tags/form" prefix="form"%>
<%@ taglib uri="http://www.springframework.org/tags" prefix="s"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Livros de Java, Android, iPhone, Ruby, PHP e muito mais -
    Casa do Código</title>
</head>
<body>
    <form:form action="${s:mvcUrl('PC#gravar').build() }" method="post"
        commandName="produto">
        <div>
            <label>Título</label> <input type="text" name="titulo" />
            <form:errors path="produto.titulo" />
        </div>
        <div>
            <label>Descrição</label>
            <textarea rows="10" cols="20" name="descricao"></textarea>
            <form:errors path="produto.descricao" />
        </div>
        <div>
            <label>Páginas</label> <input type="text" name="paginas" />
            <form:errors path="produto.paginas" />
        </div>
        <c:forEach items="${tipos}" var="tipoPreco" varStatus="status">
            <div>
                <label>${tipoPreco}</label> <input type="text"
                    name="precos[${status.index}].valor" /> <input type="hidden"
                    name="precos[${status.index}].tipo" value="${tipoPreco}" />
            </div>
        </c:forEach>
        <button type="submit">Cadastrar</button>
    </form:form>
</body>
</html>

ProdutosController.java

package br.com.casadocodigo.loja.controllers;

import java.util.List;

import javax.validation.Valid;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;
import org.springframework.validation.BindingResult;
import org.springframework.web.bind.WebDataBinder;
import org.springframework.web.bind.annotation.InitBinder;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.servlet.ModelAndView;
import org.springframework.web.servlet.mvc.support.RedirectAttributes;

import br.com.casadocodigo.loja.daos.ProdutoDAO;
import br.com.casadocodigo.loja.models.Produto;
import br.com.casadocodigo.loja.models.TipoPreco;
import br.com.casadocodigo.loja.validation.ProdutoValidation;

@Controller
@RequestMapping("/produtos")
public class ProdutosController {

    @Autowired
    private ProdutoDAO produtoDao;

    @InitBinder
    public void InitBinder(WebDataBinder binder){
        binder.addValidators(new ProdutoValidation());
    }

    @RequestMapping("/form")
    public ModelAndView form() {
        ModelAndView modelAndView = new ModelAndView("produtos/form");
        modelAndView.addObject("tipos", TipoPreco.values());

        return modelAndView;
    }

    @RequestMapping(method=RequestMethod.POST)
    public ModelAndView gravar(@Valid Produto produto, BindingResult result,  RedirectAttributes redirectAttributes){
        if(result.hasErrors()){
            return form();
        }
        produtoDao.gravar(produto);
        redirectAttributes.addFlashAttribute("message","Produto cadastrado com sucesso");
        return new ModelAndView("redirect:produtos");
    }

    @RequestMapping(method = RequestMethod.GET)
    public ModelAndView listar() {
        List<Produto> produtos = produtoDao.listar();
        ModelAndView modelAndView = new ModelAndView("produtos/lista");
        modelAndView.addObject("produtos", produtos);
        return modelAndView;
    }
}

Produto.java

package br.com.casadocodigo.loja.models;

import java.util.List;

import javax.persistence.ElementCollection;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;

@Entity
public class Produto {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private int id;
    private String titulo;
    private String descricao;
    private int paginas;
    @ElementCollection
    private List<Preco> precos;

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public List<Preco> getPrecos() {
        return precos;
    }

    public void setPrecos(List<Preco> precos) {
        this.precos = precos;
    }

    public String getTitulo() {
        return titulo;
    }

    public void setTitulo(String titulo) {
        this.titulo = titulo;
    }

    public String getDescricao() {
        return descricao;
    }

    public void setDescricao(String descricao) {
        this.descricao = descricao;
    }

    public int getPaginas() {
        return paginas;
    }

    public void setPaginas(int paginas) {
        this.paginas = paginas;
    }

    @Override
    public String toString() {
        return "Produto [titulo=" + titulo + ", descricao=" + descricao + ", paginas=" + paginas + "]";
    }

}