fix regenerate bug

app.py

@@ -258,6 +258,9 @@ def get_response(params):
         
 
 def http_bot(state, temperature, top_p, max_new_tokens):
+    if state is None:
+        return (None, "")
+    
     if state.skip_next:
         # This generate call is skipped due to invalid inputs
         yield (state, state.to_gradio_chatbot())