fix bugs

app.py

@@ -60,12 +60,15 @@ For an optimal experience, please use desktop computers for this demo, as mobile
 """
 
 def regenerate(state, image_process_mode):
-    state.messages[-1][-1] = None
-    prev_human_msg = state.messages[-2]
-    if type(prev_human_msg[1]) in (tuple, list):
-        prev_human_msg[1] = (*prev_human_msg[1][:2], image_process_mode)
-    state.skip_next = False
-    return (state, state.to_gradio_chatbot(), "", None)
+    if state is not None:
+        state.messages[-1][-1] = None
+        prev_human_msg = state.messages[-2]
+        if type(prev_human_msg[1]) in (tuple, list):
+            prev_human_msg[1] = (*prev_human_msg[1][:2], image_process_mode)
+        state.skip_next = False
+        return (state, state.to_gradio_chatbot(), "", None)
+    else:
+        return (None, "", "", None)
 
 
 def clear_history():