Summary: This reducer was mutating existing state, which will throw since we freeze state to prevent exactly that. Fixed it by using a defensive copy. Note that `newActiveNotifications` doesn't suffer from the same as it is freshly allocated. See https://www.internalfb.com/intern/diffusion/FBS/browse/master/xplat/sonar/desktop/app/src/reducers/notifications.tsx?commit=29b592866b44&lines=160 Reviewed By: passy Differential Revision: D26399200 fbshipit-source-id: 5f9f22db3de48f4f595a028faf9e9ec547641cfb