FIX: Handle edge case in UserPostsStream#findItems when loading (#35952)

Avoid throwing an error in `UserPostsStream#findItems` if already
loading or unable to load more items. This improves resilience and
prevents potential uncaught exceptions during certain edge cases in user
post streams.

frontend/discourse/app/models/user-posts-stream.js

@@ -34,7 +34,7 @@ export default class UserPostsStream extends EmberObject {
 
   async findItems() {
     if (this.loading || !this.canLoadMore) {
-      throw new Error();
+      return;
     }
 
     this.loading = true;