Thymeleaf error : An error happened during template parsing - spring

Question

I don't know what is wrong with my view but it is impossible for it to work, the view always gives an error even if idStudent is set correctly.I don't know if there is also another way to do it, but thymeleaf always fail
here is the error - > Exception evaluating SpringEL expression: "student.id == {idStudent}" (template: "/content/course/list-course-student" - line 55, col 24)
here is my controller
#GetMapping("/list-courses-student")
public String listCoursesByUser(#RequestParam(required = false) int page,Model model) {
User currentUser = userService.findByName(this.currentUser.getUsername());
int numberPages = coursesService.getNumberPages();
List<Course> courses = coursesService.findAllCourses(page,true);
model.addAttribute("coursesDisponibles",courses);
model.addAttribute("numberPages",numberPages);
model.addAttribute("currentPage",page);
model.addAttribute("idStudent",currentUser.getStudent().getId());
return "/content/course/list-course-student";
}
My view
<div class="card-body table-responsive p-0">
<table class="table table-hover text-nowrap">
<thead>
<tr>
<th>Profesor</th>
<th>Nombre</th>
<th>Description</th>
<th>Code</th>
<th>Inicio</th>
<th>Final</th>
<th>Acciones</th>
</tr>
</thead>
<tbody>
<th:block th:each="course : ${coursesDisponibles}">
<tr>
<td
th:text="${course.teacher == null} ? 'No Asignado' : ${course.teacher.nombre}"></td>
<td th:text="${course.nombre}"></td>
<td th:text="${course.descripcion}"></td>
<td th:text="${course.code}"></td>
<td
th:text="${#temporals.format(course.inicioDate, 'dd/MM/yyyy')}"></td>
<td
th:text="${#temporals.format(course.finalDate, 'dd/MM/yyyy')}"></td>
<td>
<th:block th:each="student : course.Estudiantes">
<th:block th:if="${student != null}">
<th:block th:if="${student.id == {idStudent}}">
<a class="btn btn-success btn-sm" href="">Añadido</a>
<a class="fas fa-inbox ml-3"
th:href="${'/tareas/list-tareas-student?page=1&course=' + {course.id}}"></a>
</th:block>
</th:block>
<th:block th:unless="${student == null}">
<a
th:classappend="${course.teacher == null} ? 'btn btn-primary btn-sm disabled':'btn btn-primary btn-sm' "
th:href="${'/courses/add?idCourse=' + {course.id} + '&idStudent=' + {idStudent}}">Añadir</a>
</th:block>
</th:block>
</td>
</th:block>
</tr>
</tbody>
</table>
</div>

Answer 1

The exception you're most likely experience is NullPointerException, because I think you have a wrong th:each declaration:
<th:block th:each="student : course.Estudiantes">
instead of
<th:block th:each="student : ${course.Estudiantes}">
Thus, expression th:if=${student.id == idStudent} throws NullPointerException as it tries to access the id property on a null student object.
Also note that in order to preventer other NPE's you can use a ? operator in your expressions, like this: ${student?.id}.