Agree an update like this it might be possible that persistence isn’t working quite as expected. If lasUpdate is returning null for any of the members that line will fail.
You can filter those out
call lastItem = Windows.members.filter[w | w.lastUpdate != null].sortBy...
Though since you are on the snapshot you should be using Member of and triggering item if you are using this line to find what item triggered the rule. Features for Rules that work with Groups