FIX: Do not error _removeDeleteOnOwnerReplyBookmarks on navigate (#14426)

When creating a reply after already navigating out of the
topic (e.g. open the reply composer, go to a different topic,
then create the post), the _removeDeleteOnOwnerReplyBookmarks
function was erroring because it relied on the topic model
being present.

We can skip this function altogether if the topic model is _not_
present, because the PostCreator already takes care of deleting
bookmarks with the on_owner_reply auto_delete_preference. The
_removeDeleteOnOwnerReplyBookmarks function just cleans up the
in-memory post stream and topic model.

app/assets/javascripts/discourse/app/controllers/topic.js

@@ -211,6 +211,14 @@ export default Controller.extend(bufferedProperty("model"), {
   },
 
   _removeDeleteOnOwnerReplyBookmarks() {
+    // the user has already navigated away from the topic. the PostCreator
+    // in rails already handles deleting the bookmarks that need to be
+    // based on auto_delete_preference; this is mainly used to clean up
+    // the in-memory post stream and topic model
+    if (!this.model) {
+      return;
+    }
+
     const posts = this.get("model.postStream.posts");
     if (posts) {
       posts