NLPO Code Error and Query About gymnasium vs gym Usage

I hope this message finds you well. I am writing to report an issue I encountered in the NLPO project that you maintain on GitHub.

While executing the following block of code， in lines 278 of nlpo.py:
if eval_env is not None and self.seed is not None:
eval_env.seed(self.seed)
eval_env = self._get_eval_env(eval_env)
if not self._custom_logger:

I got:eval_env = self._get_eval_env(eval_env)
AttributeError: 'NLPO' object has no attribute '_get_eval_env'.

I also found that no code related to '_get_eval_env'. in this repository, did i miss some key files? IF yes please let me know.

In addition to this, I have a query regarding library usage. I would like to know if it's possible to use 'gymnasium' in place of 'gym' within the NLPO project. If so, could you please guide on how to substitute all relevant 'gym' imports to 'gymnasium'?