How would one ensure destructor call of a Singleton, which might be used in Qml or not?

[c64zottel]

A Singleton to use in QML is registered via qmlRegisterSingletonType. The Qml engine takes responsible to call the destructor thats why we instanciate the Singleton somewhat like this:

static QObject * instance(QQmlEngine *, QJSEngine *) {
    static UserInputManagement * uim = 0;
    if( not uim ) uim = new UserInputManagement;
    return uim;
}

But if we want to use such a Singleton as gate to a library than we don't know if the client uses it in a QML context or not.

How to ensure the destructor is called, either way?