BUG: Fix Slicer crash due to wrong value return in AddModel
Created by: agirault
vtkSlicerModelsLogic::AddModel, if the call to the storageNode
ReadData() failed, an error message was triggered and the node
removed from the scene, but
AddModel() was still returning a pointer
modelNode, resulting in Slicer crashing.
The bug was encountered when loading a VTK Volume (DATASET set to STRUCTURED_POINTS) and not selecting the 'Volume' description, since the default description for the legacy VTK files is 'Model' in Slicer.