QQmlEngine.contextForObject always returns None

void MyCppItem::init()
{
    QQmlEngine *engine = qmlEngine(this);
    // Or:
    // QQmlEngine *engine = qmlContext(this)->engine();
    QQmlComponent component(engine, QUrl::fromLocalFile("MyItem.qml"));
    QQuickItem *childItem = qobject_cast<QQuickItem*>(component.create());
    childItem->setParentItem(this);
}

import QtQuick          2.0
import QtQuick.Controls 2.2

Rectangle {
    width: 200; height: 200

    Button {
      text: "print Hello World"
      onClicked: MainWindow.Print('hello world')
    }
}

#!/bin/env python3
# -*- coding: utf-8 -*-
from PySide2.QtCore    import Slot
from PySide2.QtCore    import QUrl
from PySide2.QtQml     import qmlRegisterType
from PySide2.QtQml     import QQmlEngine
from PySide2.QtQuick   import QQuickView
from PySide2.QtQuick   import QQuickItem
from PySide2.QtWidgets import QApplication
import sys

class MainWindow(QQuickView):
    def __init__(self):
        super().__init__()
        self.setSource(QUrl('sample.qml'))
        self.rootContext().setContextProperty("MainWindow", self)
        self.show()

    @Slot('QString')
    def Print(self, value):
        print(value)
        print("context=" + str(QQmlEngine.contextForObject(self)))


if __name__ == '__main__':
    app = QApplication(sys.argv)
    w = MainWindow()
    sys.exit(app.exec_())