leaderboard server path fixes

app.py

@@ -39,13 +39,14 @@ def submit_model(task, datasets, hyp_file, submitted_by, model_id, token):
         "model_id": model_id.strip()
     }
     leaderboard_df = server.get_leaderboard()
-    existing = leaderboard_df[
-        (leaderboard_df["submitted_by"] == submitted_by.strip()) &
-        (leaderboard_df["model_id"] == model_id.strip())
-    ]
-    if not existing.empty:
-        return gr.update(value="❌ A submission with this name and model ID already exists.", visible=True), \
-            gr.update(), gr.update(selected=1), gr.update(visible=False)
+    if len(leaderboard_df) > 0:
+        existing = leaderboard_df[
+            (leaderboard_df["submitted_by"] == submitted_by.strip()) &
+            (leaderboard_df["model_id"] == model_id.strip())
+        ]
+        if not existing.empty:
+            return gr.update(value="❌ A submission with this name and model ID already exists.", visible=True), \
+                gr.update(), gr.update(selected=1), gr.update(visible=False)
 
     try:
         server.prepare_model_for_submission(hyp_file.name, metadata, task, datasets)