Fix compilation error with new VTK
Fix compilation error with new VTK

This fixes a compilation error with the GraphDialog
class that occurred after updating to the newest VTK.

Change-Id: Ibc299fe0d5fe1e73413f8ce0aee2138cff0dacb3

OpenChemistry/mongochem,OpenChemistry/mongochem,OpenChemistry/mongochem