BUG: Fix fiber bundle reader crash

[lassoan]

No description provided.

[jhlegarreta]

This is doing its job. The following tests are being fixed:

 10 - py_NsgPlanTracto (Failed)
 52 - py_TractographyExportPLY (Failed)
 54 - py_TractographyDownsample (Failed)

The remaining issues are related to the tract1 and FA nodes not being found on the scene, e.g.
https://github.com/SlicerDMRI/SlicerDMRI/actions/runs/7051030751/job/19193025928?pr=220

Related discussion:
https://discourse.slicer.org/t/fixing-extension-testing-failures/32584
#207

And the known issue of the measurement data:
#206

[ljod]

Thank you!

[ljod]

@lassoan Hi Andras! Thank you for this helpful commit! Fiber Bundles are loading now, but some of the display (specifically scalar values) seems to not be set up in the display pipeline. I described the steps to reproduce this with some example data in issue #227. Is there a change to the vtk API that we need to conform to? Or if you know what needs to be fixed, could you please let us know? Thank you!