`agent_memory` list index out of range

## Steps to reproduce

Launch server:

```bash
python -m mindsearch.app --lang en --model_format gpt4 --search_engine DuckDuckGoSearch --asy
```

Launch streamlit:

```bash
streamlit run frontend/mindsearch_streamlit.py
```

Type in a query and get this error:

```
Traceback (most recent call last):
File "/Users/cxrh/Documents/GitHub/MindSearch/mindsearch/app.py", line 142, in generate
async for message in agent(inputs, session_id=session_id):
File "/Users/cxrh/Documents/GitHub/MindSearch/mindsearch/agent/streaming.py", line 50, in __call__
async for response_message in self.forward(*message, session_id=session_id, **kwargs):
File "/Users/cxrh/Documents/GitHub/MindSearch/mindsearch/agent/mindsearch_agent.py", line 189, in forward
reference, references_url = _generate_references_from_graph(gen.ret[1])
File "/Users/cxrh/Documents/GitHub/MindSearch/mindsearch/agent/mindsearch_agent.py", line 41, in _generate_references_from_graph
assert data_item["memory"]["agent.memory"][2]["sender"].endswith("ActionExecutor")
IndexError: list index out of range
```