ok, thanks for the explanation.
Concerning the fact that the REST_Geant4_ViewGeometry macro crashes even at first try, some update: I have now a Conda environment with Root 6.26/10 (and Geant4 11.0.3).
I still have the same problem. Sometimes, I would have this crash (always with the same error message), and sometimes not, with no modification of the .gdml file between the 2 attempts.