How to properly set up a ToMany relation with Objectbox in Flutter?

Question

The Objectbox is setup fine. It works for other entities. This is the class I want to store in the Objectbox:

class A {

 String id;
 List<B> fieldA;
 List<B> fieldB;

}

The custom class B instances stored in fieldA and fieldB are all unique and won't be stored in the other field again.

My entity classes look like this:

@Entity
class AEntity{
  
  @Id(assignable: true)
  int dbId = 0;
  
  String id;
  final classBRelation1 = ToMany<CustomClassBEntity>();
  final classBRelation2 = ToMany<CustomClassBEntity>();

}

@Entity
class BEntity {
 
 @Id(assignable: true)
 int dbId = 0;

 String id;
 String field;
}

I keep getting this error: "the dbIdProperty is null" I also tried making the id not assignable and not assigning the "0" but nothing worked.