Object Binding for Thymeleaf and Spring not working

Question

I have Project object that contains the list Roles with Roleobject. I want to edit the project using the form. This form need to have checklist of roles. So I have binded Project object with th:object. Now when the user selects the check boxes and saves, it is failing with the error.

Form to render Check box

<ul class="checkbox-list">
  <li th:each="role,stat : ${allRoles}">
        <input type="checkbox" th:value="${role.id}" th:field="*{roles[__${stat.index}__].id}"/>
        <span class="primary" th:text="${role.name}"> Developer</span>
   </li>
</ul>

Project Object

public class Project {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
int id;
String name;
String description;
private String status;
private String slug;


@ManyToMany
List<Role> roles = new ArrayList<>();

@ManyToMany
List<Collaborator> collaborators = new ArrayList<>();
................
................
}

Role Object:

@Entity
public class Role {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY )
private int id;
private String name;
.............
.............
.............
}

Relevant Controller Methods

@RequestMapping("/edit-project/{slug}")
    public String editProjectFromSlug(@PathVariable String slug,Model model) {
        Project project = projectService.findProjectBySlug(slug);
        List<Role> allRoles = roleService.getAllRoles();
        //Two objects are added to the model
        model.addAttribute("project",project);
        model.addAttribute("allRoles",allRoles);
        return "edit_project";
    }

@RequestMapping(value = "/save-project", method = RequestMethod.POST)
public String saveProject(@ModelAttribute Project project,Model model){
    projectService.saveProject(project);
    return "redirect:/projects";
}

I have referred multiple answers like this. None of them are working. The code is at this github repo. Please help me to fix this error.