Hello Community, 
Was there anyone who had an issue and could give me advice on how to work around  my case described in the attached file ? 

Briefly: I set an alert checker, it checks the port state "ifOperStatus equals down" wait delay time and  sends me the message.
The port's operational status is constantly down for a long time.

Sometimes the checker get NULL for port parameters so  as  ifOperStatus is  NULL the checker defines it is not equals to "down" and sends me "false" message "Clear"

The next check time the checker gets real state  ifOperStatus equals down and as Alert was cleared last time so checker creates a new alert and sends me notification.

Every time the checker gets null I am getting Clear then Alert notification.
If I could operate with the previous state at the checker condition rule I would be set to create an alert only if the previous state was "up" AND the state for now is "down".
image.png