Fetch argument None has invalid type<class 'NoneType'>

[SuiAn-97]

First, thanks for your implementation, but sorry, I'm a high school student who just came into contact with NAS and python, and I met some questions that might be easy for you but difficult for me.
In your codes, I replaced all "K.set_session" with "tf.compat.v1.keras.backend.set_session" because the version of my tensorflow is 2.0+. Then I found that when I ran the codes, something went wrong:
Traceback: TypeError: Fetch argument None has invalid type <class 'NoneType'>

The error was related to the following codes in controller.py (function 'train_step'):
_, loss, summary, global_step = self.policy_session.run([self.train_op, self.total_loss, self.summaries_op, self.global_step], feed_dict=feed_dict)

I'm not sure if you will reply me because the question might be too simple for you, but that doesn't stop me from expressing my gratitude, and if anyone else has any suggestions, thank you very much. It will help me a lot because I can't wait to implement the code and further apply it to my own work.