Fix current_service_class method

ersilia/core/session.py

@@ -30,7 +30,7 @@ def current_identifier(self):
 
  def current_service_class(self):
  data = self.get()
- if data is None:
+ if data is None or data.get("service_class") is None:
  return None
  else:
  return data["service_class"]