Cannot read property 'sendSignalwithArg' of null

Praveen.Illa

Hi Team,
I am getting an error "Cannot read property 'sendSignalwithArg' of null" on closing of a main window.
Can someone help me in understanding why this error is coming on closing of a window.
Please find below snippet code

NewClass.h

class NewClass : public QObject
{
    Q_OBJECT
public:
    explicit NewClass(QObject *parent = nullptr);

signals:
    void sendSignalwithArg(int x);

public slots:
    void getSignalwithArg();
};

NewClass.cpp

void NewClass::getSignalwithArg()
{
    int x = 2;
    emit sendSignalwithArg(x);
}

main.cpp

int main(int argc, char *argv[])
{
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);

    QGuiApplication app(argc, argv);

    QQmlApplicationEngine engine;

    NewClass NewClassObj;
    engine.rootContext()->setContextProperty("NewClassObj", &NewClassObj);

    const QUrl url(QStringLiteral("qrc:/main.qml"));
    /*QObject::connect(&engine, &QQmlApplicationEngine::objectCreated,
                     &app, [url](QObject *obj, const QUrl &objUrl) {
        if (!obj && url == objUrl)
            QCoreApplication::exit(-1);
    }, Qt::QueuedConnection);*/
    engine.load(url);

    return app.exec();
}

main.qml

Window {
    id: mainWindow
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")

    Item {
        id: type2
        signal sendSignalwithArg(int x)
        onSendSignalwithArg: console.log("type2 - Signal recieved on QML = " + x)

        Component.onCompleted: {
            NewClassObj.sendSignalwithArg.connect(type2.sendSignalwithArg)
            NewClassObj.getSignalwithArg()
        }
        Component.onDestruction: NewClassObj.sendSignalwithArg.disconnect(type2.sendSignalwithArg)
    }
}

The output on console is
qml: type2 - Signal recieved on QML = 2
qrc:/main.qml:17: TypeError: Cannot read property 'sendSignalwithArg' of null

IntruderExcluder

Not enough code from main.cpp, but I assume that NewClass object was created after engine, so, when application unloaded from memory all variables will be freed in a backward direction of their creation. This will lead to that for a short period of time your engine will be still alive and have some bindings to an already dead object. Just try to declare your object before engine instance.

...
NewClass NewClassObj;
...
QQmlApplicationEngine engine;
engine.rootContext()->setContextProperty("NewClassObj", &NewClassObj);
...

Praveen.Illa

@IntruderExcluder

Thanks for your clarification. It resolves the error