Conform more code to strict null checking (#10167)

* Conform more code to strict null checking

* Delint

* Iterate PR based on feedback

test/useTopic-test.tsx

@@ -42,7 +42,7 @@ describe("useTopic", () => {
 
         function RoomTopic() {
             const topic = useTopic(room);
-            return <p>{topic.text}</p>;
+            return <p>{topic!.text}</p>;
         }
 
         render(<RoomTopic />);