Problem with saving an object with rest template, Spring web app - spring

Question

So I'm building a flashcard app with Spring, JPA, MySQL and using Thymeleaf.
At first when i started building I only used MVC. All functionality for saving a new user, a new flashcard and a new topic worked.
Now I connected it with a project with a restcontroller and did some modifactions.
The problem is that I can't seem to save a new flashcard (although i can save a topic and a user)
Can someone spot where i've made a mistake?
(note. i have a JPARepository in the backend)
#Controller-class
//This does not work
#PostMapping("/save")
public String set(#ModelAttribute Flashcard flashcard){
restTemplate.postForObject("http://localhost:8081/api/card/save", flashcard, Flashcard.class);
return "redirect:/";
}
//this works
#PostMapping("/savePlayer")
public String savePlayer(#ModelAttribute Player player){
restTemplate.postForObject("http://localhost:8081/api/player/save", player, Player.class);
return "homepage";
}
**#RestController-class**
#PostMapping("/api/card/save")
public Flashcard createFlashcard(#RequestBody Flashcard flashcard) {
log.info("New Card added to FlashcardRepository - {}", flashcard);
return repository.save(flashcard);
}
**Flashcard-class**
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
#Column(name = "card_id")
Long id;
String term;
String answer;
#ManyToOne
#JsonBackReference
#JoinColumn(name = "topic_id")
Topic topic;
Error message:
2022-07-15 10:17:41.555 WARN 2016 --- [nio-8081-exec-4] o.h.engine.jdbc.spi.SqlExceptionHelper : SQL Error: 1048, SQLState: 23000
2022-07-15 10:17:41.556 ERROR 2016 --- [nio-8081-exec-4] o.h.engine.jdbc.spi.SqlExceptionHelper : Column 'topic_id' cannot be null
2022-07-15 10:17:41.608 ERROR 2016 --- [nio-8081-exec-4] o.a.c.c.C.[.[.[/].[dispatcherServlet] : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is org.springframework.dao.DataIntegrityViolationException: could not execute statement; SQL [n/a]; constraint [null]; nested exception is org.hibernate.exception.ConstraintViolationException: could not execute statement] with root cause