访问QGraphicsItem的parentItem导致崩溃

Question

为什么当我尝试访问qgraphics项目的parentItem时会导致程序意外崩溃？我试图为这个问题做一个超级简单的例子，它肯定是可以重现的。我在网上找到的唯一与此崩溃有关的文章是：https://www.qtcentre.org/threads/63345-Occasional-crash-related-to-QGraphicsItem-parentItem，它谈论了我已添加但未解决问题的 prepareGeometryChange（）。

要重新创建崩溃，只需取消注释此行代码print self.parentItem()

import os, sys, uuid
from Qt import QtWidgets, QtGui, QtCore


class KnotNodeItem(QtWidgets.QGraphicsEllipseItem):
    def __init__(self, parent=None):
        super(KnotNodeItem, self).__init__(parent=parent)
        self.setAcceptHoverEvents(True)
        self.setFlags(self.ItemSendsScenePositionChanges | self.ItemIsSelectable | self.ItemIsMovable)
        self.setCursor(QtCore.Qt.PointingHandCursor)

        # call initial update to draw text with correct visuals from base class 
        self.update()


    # Overrides
    def boundingRect(self):
        rect = self.rect()
        rect.adjust(-1,-1,1,1)
        return rect


    def itemChange(self, change, value):
        if change == self.ItemScenePositionHasChanged:
            pass
            # print self.parentItem()
        return super(KnotNodeItem, self).itemChange(change, value)


    def update(self):
        self.prepareGeometryChange()
        self.setRect(-12, -12, 12*2, 12*2)
        super(KnotNodeItem, self).update()


    def paint(self, painter, option, widget=None):
        painter.save()
        painter.setRenderHint(QtGui.QPainter.Antialiasing)

        if self.isSelected():
            painter.setPen(QtGui.QPen(QtGui.QColor(255,255,255), 4, QtCore.Qt.SolidLine))
        else:
            painter.setPen(QtGui.QPen(QtGui.QColor(30,30,30), 4, QtCore.Qt.SolidLine))

        painter.setBrush(QtGui.QColor(128,128,255))
        painter.drawEllipse(self.rect())   
        painter.restore()


class MainWindowUi(QtWidgets.QMainWindow):

    def __init__(self, *args, **kwargs):
        super(self.__class__, self).__init__(*args, **kwargs)
        self.resize(400,400)

        self.scene = QtWidgets.QGraphicsScene(self)
        self.scene.setBackgroundBrush(QtGui.QBrush(QtGui.QColor(45,45,45), QtCore.Qt.SolidPattern));
        self.view = QtWidgets.QGraphicsView(self)
        self.setCentralWidget(self.view)
        self.view.setScene(self.scene)


        # Create various KnotNodeItems
        item = KnotNodeItem()
        item.setPos(300,20)
        self.scene.addItem(item)


if __name__ == '__main__':
    a = QtWidgets.QApplication(sys.argv)
    m = MainWindowUi()
    m.show()
    sys.exit(a.exec_())