train AgentInterPPO comes an error

when I use args.agent = AgentInterPPO() in StockTrading_Demo.ipynb，to train InterPPO, an error comes up when I run train_and_evaluate_mp(args):
```
Traceback (most recent call last):
File "d:\programdata\anaconda3\envs\frl\lib\multiprocessing\process.py", line 315, in _bootstrap
self.run()
File "d:\programdata\anaconda3\envs\frl\lib\multiprocessing\process.py", line 108, in run
self._target(*self._args, **self._kwargs)
File "E:\Documents\my_projects\elegant_quantitative_trading\elegant_finrl\run.py", line 431, in mp_train
obj_a, obj_c = agent.update_net(
File "E:\Documents\my_projects\elegant_quantitative_trading\elegant_finrl\agent.py", line 1046, in update_net
[
File "E:\Documents\my_projects\elegant_quantitative_trading\elegant_finrl\agent.py", line 1047, in
self.cri(buf_state[i : i + bs])
TypeError: 'NoneType' object is not callable
```