FIX: ensures stream update object is scoped to its initial topic (#1359)

* FIX: ensures stream update object is scoped to its initial topic

Before this commit you could end up in this situation where a `post-updater` is constructed for a specific topic, but the user changes topic mid steam and it ends up updating the same post number but in a  different topic as we were only checking for `post_number` and not the combination of `topic_id` + `post_number`.

* we should have the topic

Author: jjaffeux

assets/javascripts/discourse/lib/ai-streamer/updaters/post-updater.js

@@ -12,13 +12,15 @@ export default class PostUpdater extends StreamUpdater {
 
   constructor(postStream, postId) {
     super();
+
     this.postStream = postStream;
     this.postId = postId;
     this.post = postStream.findLoadedPost(postId);
+    const topicId = postStream.topic.id;
 
     if (this.post) {
       this.postElement = document.querySelector(
-        `#post_${this.post.post_number}`
+        `.topic-area[data-topic-id="${topicId}"] #post_${this.post.post_number}`
       );
     }
   }
@@ -57,6 +59,10 @@ export default class PostUpdater extends StreamUpdater {
   }
 
   async setCooked(value) {
+    if (!this.postElement) {
+      return;
+    }
+
     this.post.set("cooked", value);
 
     (await loadMorphlex()).morphInner(