fix visualization for optical system

[tontyoutoure]

put viusalisation after initialization to avoid an uncomfortable exception:

-------- WWWW ------- G4Exception-START -------- WWWW -------
*** G4Exception : modeling0015
issued by : G4PhysicalVolumeModel::Validate
Attempt to validate a volume that is no longer in the physical volume store.
*** This is just a warning message. ***
-------- WWWW -------- G4Exception-END --------- WWWW -------