`view(slicing_planes = True)` has no effect when used with `vtkXMLUnstructuredGridReader`

Here is a sample code to reproduce the issue

```Python
# https://github.com/InsightSoftwareConsortium/itkwidgets/blob/master/examples/vtkUnstructuredGrid.ipynb

import vtk
import itk
from itkwidgets import view

path_vtu = "../internal/results/internal_004_3D_plot_and_animate/results.vtu"

reader = vtk.vtkXMLUnstructuredGridReader()
reader.SetFileName(path_vtu)
reader.Update()
volume = reader.GetOutput()

view(geometries=volume, slicing_planes=True,)
```

Package versions

```Bash
(demo-notebook-EFweB4Xh-py3.7) user@debian:~/pro/code/notebooks$ pip freeze | grep vtk
pyevtk==1.1.1
vtk==9.0.1
(demo-notebook-EFweB4Xh-py3.7) user@debian:~/pro/code/notebooks$ pip freeze | grep itk
itk-core==5.1.1
itk-filtering==5.1.1
itk-meshtopolydata==0.6.2
itk-numerics==5.1.1
itkwidgets==0.32.0
```

I can provide more debug data if there is a need !

Best regards,
Joachim