I’ve noticed that FragmentActivity.activity can be null, in what situation could this occur? Can the activity be destroyed without the destruction of the fragment?

I don't think so, but a fragment can be detached from an Activity. See https://stackoverflow.com/questions/6215239/getactivity-returns-null-in-fragment-function

Can the activity be destroyed without the destruction of the fragment

Sure! Fragment detached, Activity is destroyed. It’s completely valid case

FYI, requireActivity will throw an exception but otherwise always a non null return type