Fix an error when ancestors get loaded before the status itself (#5312)

When ancestors get loaded, we scroll to the target status (i.e. skip
ancestors). However, ancestors may get loaded before the status itself,
then it causes TypeError because `this.node` is undefined yet.

Since we don't show anything until the status gets loaded, we don't need
to scroll to the target status in this time. If we get the status itslef
later, it causes `componentDidUpdate` and scrolling correctly.

app/javascript/mastodon/features/status/index.js

@@ -240,9 +240,9 @@ export default class Status extends ImmutablePureComponent {
   }
 
   componentDidUpdate () {
-    const { ancestorsIds } = this.props;
+    const { status, ancestorsIds } = this.props;
 
-    if (ancestorsIds && ancestorsIds.size > 0) {
+    if (status && ancestorsIds && ancestorsIds.size > 0) {
       const element = this.node.querySelectorAll('.focusable')[ancestorsIds.size];
       element.scrollIntoView();
     }