V9.2.0 Notification definition: undefined class name index

Hi,
Problem occurs when I want to define a notificatoion definition for an opportunity. It gives the error:
ERROR : An error occurred on 2021-06-30 at 12:31:10

On the log side, I got line:
2021-06-30 12:20:48.391 ***** ERROR ***** [V9.2.0] ERROR *****
2021-06-30 12:20:48.391 ***** ERROR ***** [V9.2.0] on file '/var/www/html/model/NotificationDefinition.php' at line (1314)
2021-06-30 12:20:48.391 ***** ERROR ***** [V9.2.0] cause = Undefined index: Opportunity_id

On php side, (with a line of mine to see the objClass Keys)
  1314         errorLog('Keys:'.implode(',',array_keys($objClass)));   <---- MY LINE TO SEE KEYS
  1315         $notif->notifiedObjectId = $objClass[$className.'_id'];^M

It seems like an upper/lower case problem. But I am reluctant to touch more php code, i am not that qualified with php...

2021-06-30 12:31:10.421 ***** ERROR ***** [V9.2.0] Keys:opportunity_id,opportunity_initialenddate,opportunity_iduser
2021-06-30 12:31:10.421 ***** ERROR ***** [V9.2.0] ERROR *****
2021-06-30 12:31:10.421 ***** ERROR ***** [V9.2.0] on file '/var/www/html/model/NotificationDefinition.php' at line (1315)
2021-06-30 12:31:10.421 ***** ERROR ***** [V9.2.0] cause = Undefined index: Opportunity_id

 

Hello,
My database is postgresql.

About the ticket i have opened, while trying to repeat it for you i figured out that, it repeats only if RULE is empty. 
So I think it is my misuage of the screen.

Any way, the related logs when rule field is empty, are as follows:
2021-07-02 14:34:51.942 ***** ERROR ***** [V9.2.0] ERROR *****
2021-07-02 14:34:51.942 ***** ERROR ***** [V9.2.0] on file '/var/www/html/model/NotificationDefinition.php' at line (1314)
2021-07-02 14:34:51.942 ***** ERROR ***** [V9.2.0] cause = Undefined index: Opportunity_id

ENTER SOMETHING AS RULE, and no log is created.