returns_stats throw TypeError

[wukan1986]

Hi,

import numpy as np
import pandas as pd
import vectorbt as vbt

close = pd.Series([1, 2, 3, 4, 5])
val_price = close
size = [1, np.nan, np.nan, np.nan, -1]
portfolio = vbt.Portfolio.from_orders(close, size,
                                      val_price=val_price,
                                      close_first=True,
                                      freq='1D')

#
print(portfolio.stats())
#
print(portfolio.returns_stats())

D:\Users\Kan\miniconda3\envs\py38_vectorbt\lib\site-packages\vectorbt\utils\datetime.py:23: RuntimeWarning: invalid value encountered in multiply
  return obj * freq_delta(freq)
Traceback (most recent call last):
  File "D:/test_vectorbt/demo_from_orders_3.py", line 16, in <module>
    print(portfolio.returns_stats())
  File "D:\Users\Kan\miniconda3\envs\py38_vectorbt\lib\site-packages\vectorbt\portfolio\base.py", line 1980, in returns_stats
    stats_obj = returns.vbt.returns.stats(**kwargs)
  File "D:\Users\Kan\miniconda3\envs\py38_vectorbt\lib\site-packages\vectorbt\returns\accessors.py", line 343, in stats
    'Duration': self.wrapper.shape[0] * self.wrapper.freq,
TypeError: unsupported operand type(s) for *: 'int' and 'NoneType'

the portfolio.stats() can get Duration 5 days 00:00:00,
but the portfolio.returns_stats() can not