BUG: Fix memory leaks associated with py_DTINotReproducibleIssue3977 test
Created by: msmolens
In a debug build running py_DTINotReproducibleIssue3977 reports that a number of VTK class instances leak.
This commit fixes the leaks by fixing incorrect memory management on an instance of vtkMRMLDiffusionTensorVolumeDisplayNode.
This may have been an inadvertent change in e4ffa1f6: https://github.com/Slicer/Slicer/commit/e4ffa1f6dcfafc1ca8f0015ecf02a0c23a8bd503#diff-e6c1e3f6844baf9d4ed1a1d2ca110320L1188