DEV: Fix a call-after-destroy issue (PR #14923)

Intersection observer callback can be called after the component gets destroyed:

Assertion Failed: calling set on destroyed object: <@ember/component:ember6019>.docked = false
    at assert (ember:37774:17)
    at _set2 (ember:17304:46)
    at Class.set (ember:29529:29)
    at Class._intersectionHandler (discourse/app/components/topic-progress:135:16)
    at Backburner._run (ember:56389:25)
    at Backburner._join (ember:56365:21)
    at Backburner.join (ember:56082:19)
    at join (ember:42874:28)
    at IntersectionObserver.eval (ember:42978:19)

GitHub

It was on my list too, I saw it the other day :+1: