Spring boot custom annotation for pathVariable

There is an annotation that should ideally throw an exception if the entity by id is not found for different controllers. Annotation:

@Target({ElementType.METHOD, ElementType.PARAMETER})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Constraint(validatedBy = CheckExistHandler.class)
public @interface CheckExist {
    Class<?> entityClass();
    String message() default "Entity with specified id does not exist!";
    Class<?>[] groups() default {};
    Class<? extends Payload>[] payload() default {};
}

validator sketches:

@Component
public class CheckExistHandler implements ConstraintValidator<CheckExist, Long> {
    @PersistenceContext
    private EntityManager entityManager;

    @Override
    public boolean isValid(Long value, ConstraintValidatorContext context) {
        if (value != 0 && entityManager.find(Topic.class, value) != null) {
            return true;
        } else {
            return false;
        }
    }
}

Method for tests from one controller:

    @GetMapping("/{topicId}")
    public ResponseEntity<TopicDto> getTopicById(@CheckExist(entityClass = Topic.class) @PathVariable("topicId") Long topicId) {
        if (!topicService.isExistByKey(topicId)) {
            throw new NotFoundException("topic not found");
        }
        return new ResponseEntity<>(topicMapper.toDto(topicService.getByKey(topicId)), HttpStatus.OK);
    }

In this regard, questions:

1. How to isolate a class from an annotation in a validator using reflection in order to correctly use the EntityManager?
2. How to throw an exception without getting 500?