public class Person implements Serializable {
    private IntegerProperty id;
    private StringProperty name;
    private SetProperty<Certificate> certificates;
    public Person() {
        id = new SimpleIntegerProperty();
    }

    public Person(String name, SetProperty<Certificate> certificates) {
        this();
        this.name = new SimpleStringProperty(name);
        this.certificates = new SimpleSetProperty<>(certificates);
    }

    public int getId() {
        return id.get();
    }

    public IntegerProperty idProperty() {
        return id;
    }

    public void setId(int id) {
        this.id.set(id);
    }

    public String getName() {
        return name.get();
    }

    public StringProperty nameProperty() {
        return name;
    }

    public void setName(String name) {
        this.name.set(name);
    }

    public ObservableSet<Certificate> getCertificates() {
        return certificates.get();
    }

    public SetProperty<Certificate> certificatesProperty() {
        return certificates;
    }

    public void setCertificates(ObservableSet<Certificate> certificates) {
        this.certificates.set(certificates);
    }
}

<class name="Person" table="PERSON">
        <id name="id" column="ID">
            <generator class="native"/>
        </id>
        <property name="name" column="NAME" type="string"/>
        <set name="certificates">
            <key column="PERSON_ID"></key>
            <one-to-many class="Certificate"/>
        </set>
    </class>

private void addPersonBtnRegisterEvents() {
        addPersonBtn.setOnAction(event -> {
            Set<Certificate> certificates = new HashSet<Certificate>();
            certificates.add(new Certificate("Senior Java Developer", new Date()));
            certificates.add(new Certificate("Junior C# Developer", new Date()));

            SetProperty<Certificate> certificateSetProperty = new SimpleSetProperty<Certificate>(FXCollections.observableSet(certificates));

            Person person = new Person(nameTextField.getText(), certificateSetProperty);


            persons.add(person);

            Session session = factory.openSession();
            Transaction transaction = session.beginTransaction();
            session.save(person);  // тут возникает IllegalArgumentException
            transaction.commit();
            session.close();
        });
    }