The observer is null if user prefers reduced motion

Turns out TypeScript is there for a reason and shouldn't be bypassed. Fixes #1135

src/shared/prerendered-app/Intro/SlideOnScroll/index.tsx

@@ -42,7 +42,7 @@ export default class SlideOnScroll extends Component<Props, State> {
     // Have to manually disconnect due to memory leaks in browsers.
     // One day we'll be able to remove this, and the private property.
     // https://twitter.com/jaffathecake/status/1405437361643790337
-    this.observer!.disconnect();
+    if (this.observer) this.observer.disconnect();
   }
 
   render({ children }: RenderableProps<Props>) {