Spring tiles Error : Cannot render an attribute that is not a string, toString returns: null

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I am making a spring mvc project and getting this error. Have checked everything multiple times but getting through it. Please have a look at my code and help me identify with where and what I am doing wrong. Have checked paths and file locations thoroughly, they are at right place.

My first page is index.jsp which is supposed to open login.jsp but I am getting this error at return "login"; org.apache.tiles.request.render.CannotRenderException: Cannot render an attribute that is not a string, toString returns: null

index.jsp :

<!DOCTYPE html>
<html lang="en">
<head>
</head>
<body>
<a href="${pageContext.request.contextPath}/register">Click here to Register</a>

</body>
</html>

register.jsp :

<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8" %>
<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>Register</title>
</head>
<body>
    <h1>Register</h1>
    <form action="${pageContext.request.contextPath}/register" method="post">
        <label>Username:</label>
        <input type="text" name="username" required><br>
        <label>Password:</label>
        <input type="password" name="password" required><br>
        <button type="submit">Register</button>
    </form>
</body>
</html>

RegisterController :

package com.jpa.springdemo.controller;

import javax.validation.Valid;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.validation.BindingResult;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.ModelAttribute;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestMapping;

import com.jpa.springdemo.entity.User;
import com.jpa.springdemo.service.UserService;

@Controller
public class RegisterController {
    
    @Autowired
    private UserService userService;
    

    @RequestMapping(value = "/")
    public String showHomePage() {
        return "home";
    }
    
    @GetMapping("/register")
    public String showRegisterForm(Model model) {
        System.err.println("get");
        model.addAttribute("user", new User());
        return "register";
    }
    
    @PostMapping("/register")

    public String processRegistrationForm(@Valid @ModelAttribute("user") User user, BindingResult result) {
        if (result.hasErrors()) {
            return "register";
        }
        userService.save(user);
        return "home";
    }
    
}

Tiles.xml :

<?xml version="1.0" encoding="UTF-8" ?>

<!DOCTYPE tiles-definitions PUBLIC "-//Apache Software Foundation//DTD Tiles Configuration 3.0//EN" "http://tiles.apache.org/dtds/tiles-config_3_0.dtd">

<tiles-definitions>
    <!-- Layout for Public pages -->
    
<definition name="home">
    <put-attribute name="title" value="Home" />
    <put-attribute name="body" value="/WEB-INF/views/index.jsp" />
</definition>


    <definition name="login">
        <put-attribute name="title" value="Login" />
        <put-attribute name="body" value="/WEB-INF/views/login.jsp" />
    </definition>

<definition name="register">
    <put-attribute name="title" value="Register" />
    <put-attribute name="body" value="/WEB-INF/views/register.jsp" />
</definition>

</tiles-definitions>

WebConfig :

package com.jpa.springdemo.config;

import java.util.Locale;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.support.ReloadableResourceBundleMessageSource;
import org.springframework.core.env.Environment;
import org.springframework.transaction.annotation.EnableTransactionManagement;
import org.springframework.web.multipart.commons.CommonsMultipartResolver;
import org.springframework.web.servlet.config.annotation.DefaultServletHandlerConfigurer;
import org.springframework.web.servlet.config.annotation.EnableWebMvc;
import org.springframework.web.servlet.config.annotation.InterceptorRegistry;
import org.springframework.web.servlet.config.annotation.ResourceHandlerRegistry;
import org.springframework.web.servlet.config.annotation.ViewControllerRegistry;
import org.springframework.web.servlet.config.annotation.ViewResolverRegistry;
import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;
import org.springframework.web.servlet.i18n.CookieLocaleResolver;
import org.springframework.web.servlet.i18n.LocaleChangeInterceptor;
import org.springframework.web.servlet.view.InternalResourceViewResolver;
import org.springframework.web.servlet.view.JstlView;
import org.springframework.web.servlet.view.tiles3.TilesConfigurer;
import org.springframework.web.servlet.view.tiles3.TilesViewResolver;

@EnableWebMvc
@Configuration
@EnableTransactionManagement
@ComponentScan(basePackages = "com.jpa.springdemo")
public class WebConfig implements WebMvcConfigurer {

    @Autowired
    Environment environment;
    /*
     * @Bean public ViewResolver getViewResolver() { InternalResourceViewResolver
     * resolver = new InternalResourceViewResolver();
     * resolver.setPrefix("/WEB-INF/views/"); resolver.setSuffix(".jsp"); return
     * resolver; }
     */

    @Bean
    public TilesConfigurer tilesConfigurer() {
        TilesConfigurer tilesConfigurer = new TilesConfigurer();
        tilesConfigurer.setDefinitions(new String[] { "/WEB-INF/tiles-defs/tiles.xml" });
        tilesConfigurer.setCheckRefresh(true);

        return tilesConfigurer;
    }

    @Override
    public void configureViewResolvers(ViewResolverRegistry registry) {
        TilesViewResolver viewResolver = new TilesViewResolver();
        registry.viewResolver(viewResolver);
    }

    @Override
    public void addResourceHandlers(ResourceHandlerRegistry registry) {
        registry.addResourceHandler("/resources/**").addResourceLocations("/resources/");
    }

    @Bean(name = "multipartResolver")
    public CommonsMultipartResolver multipartResolver() {
        CommonsMultipartResolver multipartResolver = new CommonsMultipartResolver();
        //multipartResolver.setMaxUploadSize(1000000);
        return multipartResolver;
    }

    @Bean(name = "localeResolver")
    public CookieLocaleResolver getCookieLocaleResolver() {
        CookieLocaleResolver localeResolver = new CookieLocaleResolver();
        localeResolver.setCookieName("myAppLocaleCookie");
        // localeResolver.setDefaultLocale(Locale.forLanguageTag("hi_IN"));
        localeResolver.setDefaultLocale(new Locale("hi", "IN"));
        localeResolver.setCookieMaxAge(3600);

        return localeResolver;
    }

    @Bean(name = "localeInterceptor")
    public LocaleChangeInterceptor getLocaleInterceptor() {
        LocaleChangeInterceptor interceptor = new LocaleChangeInterceptor();
        interceptor.setParamName("lang");
        return interceptor;
    }

    @Override
    public void addInterceptors(InterceptorRegistry registry) {
        registry.addInterceptor(getLocaleInterceptor());
    }
    
    /*
     * @Override public void addViewControllers(ViewControllerRegistry registry) {
     * registry.addViewController("/").setViewName("index"); }
     */
    @Bean(name = "messageSource")
    public ReloadableResourceBundleMessageSource messageSource() {
        ReloadableResourceBundleMessageSource messageBundle = new ReloadableResourceBundleMessageSource();
        messageBundle.setBasenames("classpath:i18n/Transactions", "classpath:i18n/ValidationMessages");
        messageBundle.setDefaultEncoding("UTF-8");
        return messageBundle;
    }

    @Override
    public void configureDefaultServletHandling(DefaultServletHandlerConfigurer configurer) {
        configurer.enable();
    }

    @Bean
    public InternalResourceViewResolver viewResolver() {
        InternalResourceViewResolver viewResolver = new InternalResourceViewResolver();
        viewResolver.setViewClass(JstlView.class);
        viewResolver.setPrefix("/WEB-INF/views/");
        viewResolver.setSuffix(".jsp");

        return viewResolver;
    }

}

tiles dependency in pom.xml

        <dependency>
  <groupId>org.apache.tiles</groupId>
  <artifactId>tiles-core</artifactId>
  <version>3.0.8</version>
</dependency>

<dependency>
  <groupId>org.apache.tiles</groupId>
  <artifactId>tiles-core</artifactId>
  <version>3.0.8</version>
</dependency>
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