message: FIX: refactored undefined variable in get_system
diff: @@ -60,5 +60,5 @@ class Descriptor(with_metaclass(ABCMeta)):
  return System.from_atoms(system)
  else:
  raise ValueError(
  - "Invalid system with type: '{}'.".format(system_type)
  + "Invalid system with type: '{}'.".format(type(system))
  )
  