attempted to assign id from null one-to-one property

@Entity
public class Prima implements Serializable {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    Long id;

    //@PrimaryKeyJoinColumn
    @OneToOne(mappedBy = "parent", optional = true , cascade = CascadeType.ALL)
    Secunda optionalData;
}

@Entity
public class Secunda implements Serializable {

    @Id
    @Column(name = "id", nullable = false)
    Long id;

    @MapsId
    @OneToOne(optional = false)
    @JoinColumn(name = "id", nullable = false)
    Prima parent;
}

class Test {
   Long primaId;
   void beforTest() {
        Prima prim = new Prima();
        prim.setOptionalData(null); // <-- null
        prim = repoPrima.save(prim);
       this.primaId = prim.getId();
   }

   void test() {
        // we emulate object recieved from json
        Prima prim = new Prima();
        prim.setId(this.primaId);
        {
            Secunda sec = new Secunda();
            sec.setParent(prim); // <-- not null ! ! !
            prim.setOptionalData(sec);
        }

        Prima savedPrima = repoPrima.save(prim); // <-- exception here
  }
}