Another fix

src/main/resources/templates/edition/student.html

@@ -112,7 +112,7 @@
                         </div>
                     </div>
 
-                    <div class="flex justify-end">
+                    <div class="flex justify-end" th:unless="${group == null}">
                         <button
                             th:if="${@authorizationService.canLeaveGroup(group.id)}"
                             class="button"