Improve performance of widget startup slightly

See https://github.com/matrix-org/matrix-react-sdk/pull/6733#pullrequestreview-745513275

src/stores/widgets/StopGapWidget.ts

@@ -301,7 +301,8 @@ export class StopGapWidget extends EventEmitter {
         // requests timeline capabilities in other rooms down the road. It's just easier to manage here.
         for (const room of MatrixClientPeg.get().getRooms()) {
             // Timelines are most recent last
-            const roomEvent = arrayFastClone(room.getLiveTimeline()?.getEvents() || []).reverse()[0];
+            const events = room.getLiveTimeline()?.getEvents() || [];
+            const roomEvent = events[events.length - 1];
             if (!roomEvent) continue; // force later code to think the room is fresh
             this.readUpToMap[room.roomId] = roomEvent.getId();
         }