vtk render with distancewidget inside a PyQT application

the following simple code does not work and I just want to use QVTKRenderWindowInteractor with vtkDistanceWidget to measure sphere length with mouse. but vtkDistanceWidget does not work

import sys
import vtk
from PyQt5 import QtCore, QtWidgets
from vtk.qt.QVTKRenderWindowInteractor import QVTKRenderWindowInteractor

class MainWindow(QtWidgets.QMainWindow):

    def __init__(self, parent = None):
        QtWidgets.QMainWindow.__init__(self, parent)

        self.frame = QtWidgets.QFrame()

        self.vl = QtWidgets.QVBoxLayout()
        self.vtkWidget = QVTKRenderWindowInteractor(self.frame)
        self.vl.addWidget(self.vtkWidget)

        self.ren = vtk.vtkRenderer()
        self.vtkWidget.GetRenderWindow().AddRenderer(self.ren)
        self.iren = self.vtkWidget.GetRenderWindow().GetInteractor()

        widget = vtk.vtkDistanceWidget()
        widget.SetInteractor(self.iren)
        widget.CreateDefaultRepresentation()
        widget.On()

        # Create source
        source = vtk.vtkSphereSource()
        source.SetCenter(0, 0, 0)
        source.SetRadius(5.0)

        # Create a mapper
        mapper = vtk.vtkPolyDataMapper()
        mapper.SetInputConnection(source.GetOutputPort())

        # Create an actor
        actor = vtk.vtkActor()
        actor.SetMapper(mapper)

        self.ren.AddActor(actor)

        self.ren.ResetCamera()

        self.frame.setLayout(self.vl)
        self.setCentralWidget(self.frame)

        self.show()
        self.iren.Initialize()


if __name__ == "__main__":

    app = QtWidgets.QApplication(sys.argv)

    window = MainWindow()

    sys.exit(app.exec_())