Add check for logged in user before accessing

app.py

@@ -81,6 +81,8 @@ def groupby_hf_user_papers(papers, hf_user_name):
 def get_papers(
     author_name: str, positive_arxiv_ids: str, hf_user_name: Optional[gr.OAuthProfile]
 ):
+    if not hf_user_name:
+        raise gr.Error("You must be logged in to use this Space")
     hf_user_name = hf_user_name.preferred_username
     positive_arxiv_ids = positive_arxiv_ids.split(",")
     potential_authors = query_author(author_name)