Binding is not working when 'value' goes to null.

#include <QGuiApplication>
#include <QQuickView>
#include <QQmlContext>

class MyView: public QQuickView
{
    Q_OBJECT
    Q_PROPERTY(QObject *test READ test WRITE setTest NOTIFY testChanged)

public:
    void init() { rootContext()->setContextProperty(QLatin1String("myWindow"), this); }
    QObject *test() { return m_test; }
    void setTest(QObject *t) { qInfo() << "setTest" << t << m_test; m_test = t; emit testChanged();}

signals:
    void testChanged();

private:
    QObject *m_test = nullptr;
};

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

    QGuiApplication app(argc, argv);

    const QUrl url(QStringLiteral("qrc:/main.qml"));

    MyView view;
    view.init();
    view.setSource(url);
    view.show();

    return app.exec();
}

#include "main.moc"

import QtQuick 2.12
import QtQuick.Window 2.12

Rectangle {
    id: root
    visible: true
    width: 640
    height: 480    
    color: "red"
    property var myValue: -1


    MouseArea {
        anchors.fill: parent
        onClicked: { root.toggle(); }
    }


    function toggle() {
        if (root.myValue == null) {
            root.color = "yellow"
            console.log("set to root")
            root.myValue = root
        } else {
            root.color = "red"
            console.log("set to null")
            root.myValue = null
        }
    }

    Item {
        id: qmlWindow
        property var test: null
        onTestChanged: console.log("qmlWindow", test)
    }

    Binding {
        target: myWindow
        property: "test"
        value: root.myValue
    }

    Binding {
        target: qmlWindow
        property: "test"
        value: root.myValue
    }
}