Demonstrate how QQmlContext's property may be garbage collected before QML component gets destroyed and leads to "TypeError: Cannot read property 'foobar' of null" in PySide2

demo.py

import sys
from PySide2 import QtCore, QtGui, QtQuick


class Data(QtCore.QObject):
    color = QtCore.Property(str, fget=lambda _: "purple", constant=True)


if __name__ == '__main__':

    class Application(QtGui.QGuiApplication):
        def __init__(self):
            super(Application, self).__init__(sys.argv)

            window = QtQuick.QQuickView()

            data = Data()  # wrong, no parent.
            # data = Data(parent=window)  # correct
            # ^^^ Uncomment above lines to compare the result on quit.

            engine = window.engine()
            engine.addImportPath(".")
            context = engine.rootContext()
            context.setContextProperty("DATA", data)

            window.setSource(QtCore.QUrl.fromLocalFile("demo.qml"))
            window.show()

            self.window = window
            # self.data = data
            # ^^^ We could keep a reference here, but it still may die before QML,
            #     so the error will be random and hard to spot in this example.

    app = Application()
    app.exec_()

demo.qml

import QtQuick 2.0

Rectangle {
    anchors.fill: parent
    color: DATA.color
    // Will raise "TypeError: Cannot read property 'color' of null",
    // if `DATA` has no parent assigned and become null object.
}

Some references:
https://doc.qt.io/qt-5/qqmlcontext.html#details
https://wiki.qt.io/Defining-and-using-constants-from-PySide-in-QML