Fix race condition in post preview

The post composer could have been closed in between scheduling and
executing the callback.

Fixes flarum/org#58.
Refs #1881.

framework/core/js/src/forum/components/ReplyPlaceholder.js

@@ -52,6 +52,10 @@ export default class ReplyPlaceholder extends Component {
     // body with a preview.
     let preview;
     const updateInterval = setInterval(() => {
+      // Since we're polling, the composer may have been closed in the meantime,
+      // so we bail in that case.
+      if (!app.composer.component) return;
+
       const content = app.composer.component.content();
 
       if (preview === content) return;