"IndexError: list index out of range" in examples/dqn_cartpole.py 

I run the program "examples/dqn_cartpole.py ", but there was error.
The error is that:
```
IndexError: list index out of range
```

According to message, that happens in line 46.
```
dqn.fit(env, nb_steps=50000, visualize=True, verbose=2)
```

I'm using tensorflow 2.3.0. Is that cause of that error?
By the way, version of Keras-RL2 is 1.0.4.