TypeError: unsupported operand type(s) for -: 'NoneType' and 'Parameter'

~/anaconda3/envs/ai4finance/lib/python3.8/site-packages/elegantrl/agents/AgentBase.py in explore_one_env(self, env, horizon_len, if_random)
91 get_action = self.act.get_action
92 for t in range(horizon_len):
---> 93 action = torch.rand(1, self.action_dim) * 2 - 1.0 if if_random else get_action(state)
94 states[t] = state
95

~/anaconda3/envs/ai4finance/lib/python3.8/site-packages/elegantrl/agents/net.py in get_action(self, state)
200
201 def get_action(self, state: Tensor) -> Tensor: # for exploration
--> 202 state = self.state_norm(state)
203 action = self.net(state).tanh()
204 noise = (torch.randn_like(action) * self.explore_noise_std).clamp(-0.5, 0.5)

~/anaconda3/envs/ai4finance/lib/python3.8/site-packages/elegantrl/agents/net.py in state_norm(self, state)
184
185 def state_norm(self, state: Tensor) -> Tensor:
--> 186 return (state - self.state_avg) / self.state_std
187
188