class Application : public QObject  {   
public:
    Q_OBJECT
    //QML_ELEMENT           // prevents from using the State in when clause of QML states at all
    QML_NAMED_ELEMENT(App)  // ok, but emits an error in QML

    enum State { OFF, ON, };  // using enum class doesn't make a difference
    Q_ENUM(State)
    
    Q_PROPERTY(Application::State state READ state NOTIFY stateChanged)

Application()  {
    auto offState = new QState(&m_stateMachine);
    offState->assignProperty(&m_stateMachine, g_state_id, State::OFF);
    QObject::connect(offState, &QState::entered, this, &Application::setState);

    auto onState = new QState(&m_stateMachine);
    onState->assignProperty(&m_stateMachine, g_state_id, State::ON);
    QObject::connect(onState, &QState::entered, this, &Application::setState);

    offState->addTransition(this, &Application::toggle, onState);
    onState->addTransition(this, &Application::toggle, offState);

    m_stateMachine.setInitialState(offState);
    m_stateMachine.start();
}

Application::State state() const {
    return static_cast<Application::State> (m_stateMachine.property("state").toInt());
}
bool isState(Application::State theState) const {
    return state() == theState;
}

signals:
   void stateChanged();
   void toggle();

private:
    QStateMachine m_stateMachine;
    State m_state;
    
    void Application::setState() {
       if (state() == m_state) return;    
       m_state = state();    
       emit stateChanged(); 
    }
};    

In my RowLayout element, I want to use the state like so:

// ... inside some RowLayout target onOffBtn is a simple Button
states : [
    State {
    	name: "off"
    	//when: app.state === 0               // Ok
        //when: app.isState(App.OFF)     // not working at all (0 in all cases)
    	when: app.state === App.OFF   // TypeError: Cannot read property 'state' of null
    	PropertyChanges { target: onOffBtn; text: "turn ON"; }
    },
    State {
    	name: "on"
    	//when: app.state === 1               // Ok
    	//when: app.isState(App.ON)       // not working at all   	when: app.state === App.ON
    	PropertyChanges { target: onOffBtn; text: "turn OFF"; }
    }
]

Using the enum state from the registered type App, I get an error about the property state being null. But the code is working, the when part is evaluated correctly.
Using the enum in another context (QML Button), like

onClicked : {
     if (app.isState(App.OFF))
          console.log("app.isState(App.OFF)")
     else if (app.isState(App.ON))
          console.log("app.isState(App.ON)")

     if (app.state === App.OFF)
          console.log("app.state === App.OFF")
     else if (app.state === App.ON)
          console.log("app.state === App.ON")
}

onClicked produces the expected outputs without errors or warnings.

I expected a much better way coupling the C++ SM with QML, one without using enums to react on the current state in QML (not using the QML StateMachine type).

Am I using the states / property binding the wrong way? What is the suggested way to handle the current C++ SM state in QML?