Conform more code to strict null checking (#10167)

* Conform more code to strict null checking

* Delint

* Iterate PR based on feedback

src/components/views/user-onboarding/UserOnboardingHeader.tsx

@@ -30,7 +30,7 @@ const onClickSendDm = (ev: ButtonEvent): void => {
 };
 
 interface Props {
-    useCase: UseCase;
+    useCase: UseCase | null;
 }
 
 export function UserOnboardingHeader({ useCase }: Props): JSX.Element {

src/components/views/user-onboarding/UserOnboardingPage.tsx

@@ -38,7 +38,7 @@ interface Props {
 // We decided to only show the new user onboarding page to new users
 // For now, that means we set the cutoff at 2022-07-01 00:00 UTC
 const USER_ONBOARDING_CUTOFF_DATE = new Date(1_656_633_600);
-export function showUserOnboardingPage(useCase: UseCase): boolean {
+export function showUserOnboardingPage(useCase: UseCase | null): boolean {
     return useCase !== null || MatrixClientPeg.userRegisteredAfter(USER_ONBOARDING_CUTOFF_DATE);
 }
 
@@ -55,13 +55,11 @@ export function UserOnboardingPage({ justRegistered = false }: Props): JSX.Eleme
     const [showList, setShowList] = useState<boolean>(false);
     useEffect(() => {
         if (initialSyncComplete) {
-            let handler: number | null = window.setTimeout(() => {
-                handler = null;
+            const handler = window.setTimeout(() => {
                 setShowList(true);
             }, ANIMATION_DURATION);
             return () => {
                 clearTimeout(handler);
-                handler = null;
             };
         } else {
             setShowList(false);