vtkSliderWidget doesn't display in my render window python

Hi
I have four QVTKRenderWindowInteractor and wants to add a slider into 3 of them.
These sliders help to slide through slices.
I cannot insert the slider into my render windows.
I get this Error :


and Here is my code :

    render_win=QVTKRenderWindowInteractor()
    interactor = render_win.GetRenderWindow().GetInteractor()

    self.sliderRep = vtk.vtkSliderRepresentation3D()
    self.sliderRep.SetMinimumValue(0)
    self.sliderRep.SetMaximumValue(self.slices_number)
    self.sliderRep.SetValue(0)

    self.sliderRep.GetPoint1Coordinate().SetCoordinateSystemToNormalizedDisplay()
    self.sliderRep.GetPoint1Coordinate().SetValue(0.9, 0.1)
    self.sliderRep.GetPoint2Coordinate().SetCoordinateSystemToNormalizedDisplay()
    self.sliderRep.GetPoint2Coordinate().SetValue(0.9, 0.9)
    self.sliderRep.SetSliderLength(0.075)  
    self.sliderRep.SetSliderWidth(0.05)
    self.sliderRep.SetEndCapLength(0.05)  
    self.sliderRep.BuildRepresentation()

    sliderWidget = vtk.vtkSliderWidget()
    sliderWidget.SetInteractor(interactor)
    sliderWidget.SetRepresentation(self.sliderRep)
    sliderWidget.SetAnimationModeToAnimate()
    sliderWidget.EnabledOn()
    sliderWidget.SetCurrentRenderer(widget.renderer)

    sliderWidget.AddObserver("InteractionEvent", partial(self.slider_update_call_back, vtkObj=self.sliderRep))
    render_win.GetRenderWindow().Render()
    interactor.Start()

and my UI is like this :

Thank you for your helps.