Eliminate recurring message "ERROR SCHEDULER-423 There is no supervisor"

If there is no supervisor then the following message appears frequently:

[ERROR SCHEDULER-423 There is no supervisor]
_set_excepinfo("SCHEDULER-423 There is no supervisor","sos::scheduler::com_objects::Com_spooler::get_Supervisor_client)
_set_excepinfo("Spooler.Spooler","SCHEDULER-423 There is no supervisor (Calling sos::scheduler::com_objects::Com_spooler::get_Supervisor_client))
.[xc.insert 3, "SCHEDULER-423 There is no supervisor (Calling sos::scheduler::com_objects::Com_spooler::get_Supervisor_client)"]

The recurring message has to be eliminated.

The message appears if the api-method "spooler.supervisor_client()" is called.

TODO:
If no supervisor ist existent "spooler.supervisor_client()" should return null.
No message is to be printed and no exception is to be thrown.