Update app.py

Return error message

app.py

@@ -65,7 +65,7 @@ def generate_response(question: str) -> list[str, str, str]:
     global dspy_rag_model
 
     if dspy_rag_model is None:
-        return "RAG model not built. Please build the model first."
+        return "RAG model not built. Please build the model first.", [], []
 
     # Generate response
     responses, references, snippets = dspy_rag_model.generate_response(