fix(status_list): Guard against missing ref (#4353)

app/javascript/mastodon/components/status_list.js

@@ -31,6 +31,7 @@ export default class StatusList extends ImmutablePureComponent {
   intersectionObserverWrapper = new IntersectionObserverWrapper();
 
   handleScroll = debounce(() => {
+    if (this.node) {
       const { scrollTop, scrollHeight, clientHeight } = this.node;
       const offset = scrollHeight - scrollTop - clientHeight;
       this._oldScrollPosition = scrollHeight - scrollTop;
@@ -42,6 +43,7 @@ export default class StatusList extends ImmutablePureComponent {
       } else if (this.props.onScroll) {
         this.props.onScroll();
       }
+    }
   }, 200, {
     trailing: true,
   });