qml: ReferenceError: serial is not defined

qml编译警告：qrc:/Form02.qml:67: ReferenceError: serial is not defined
serial是我自定义的一个串口线程，在编译时报警告说serial未定义，但又能正常访问获取数据，使用代码示例如下：

示例代码：

{
    //1
    ui->quickWidget_2->setSource(QUrl("qrc:/Form02.qml"));
    //2
    serial = new SerialPort;
    ui->quickWidget_2->rootContext()->setContextProperty("serial", serial);
    ui->quickWidget_2->setResizeMode(QQuickWidget::SizeRootObjectToView);
    //3 cpp 与 qml信号连接
    QObject* obj = ui->quickWidget_2->rootObject();
    if(QMetaObject::invokeMethod(obj, "slotOpenSerial", Qt::DirectConnection, Q_ARG(bool, true), Q_ARG(QString, "")))
        connect(serial, SIGNAL(openMsg(bool,QString)), obj, SIGNAL(slotOpenSerial(bool,QString)));
}

解决办法：先设置串口对象在根节点中的name,再将qml添加到quickWidget

{
    //1
    serial = new SerialPort;
    ui->quickWidget_2->rootContext()->setContextProperty("serial", serial);
    ui->quickWidget_2->setResizeMode(QQuickWidget::SizeRootObjectToView);
    //2
    ui->quickWidget_2->setSource(QUrl("qrc:/Form02.qml"));
    //3 cpp 与 qml信号连接
    QObject* obj = ui->quickWidget_2->rootObject();
    if(QMetaObject::invokeMethod(obj, "slotOpenSerial", Qt::DirectConnection, Q_ARG(bool, true), Q_ARG(QString, "")))
        connect(serial, SIGNAL(openMsg(bool,QString)), obj, SIGNAL(slotOpenSerial(bool,QString)));
}