"is valid" remains true after actor destroy

I think i need some help to understand how “is valid” works. I have two small blueprints in my testlevel.
What i tried to achieve is to let the first blueprint rotate as long as the second exists.

I used this code for the rotating object:

The “Target” variable is set in the Level to the second blueprint and if i run the game it starts rotating as it should.
Then i remove the second bluprint (I use trace and destroy actor for this) and it works fine.
The object is removed from the level and from the actor list.
But the problem i have is, the “Target” reference stays valid for about 60 seconds after i removed the second object. This means my “is valid” check stays true and the object keeps rotating.
Is there a way to let the is valid check become false immediatelly after the actor was removed?
Thank you.