Update app.py

app.py

@@ -680,6 +680,11 @@ def package_and_export(
     vocal_silence_thresh_db,
     vocal_min_silence_len_ms
 ):
+    # --- FIX: Check if Phase 1 was run ---
+    if not track_folder_str:
+        raise gr.Error("Phase 1 incomplete! Please run 'Separate Stems' first, or wait for it to finish.")
+    # -------------------------------------
+
     try:
         track_folder = Path(track_folder_str)
         bpm = int(bpm)