How do I use "If window doesnt exist" ?

[JakkFish]

I'm trying to preform an action after a check to make sure the window doesn't exist how would I go about doing that? I haven't used this feature before, And I tried this in the picture and nothing would happen

[JakkFish]

i'm not sure how to word this correctly but I'd appreciate if someone could help me if they know what I'm trying to say