使用当组合策略模块运行模拟盘时，不管是盘中还是盘后，都会出现错信息：
Traceback (most recent call last):
File "C:\vnstudio\lib\site-packages\vnpy\app\portfolio_strategy\engine.py", line 324, in call_strategy_func
func(params)
File "C:\vnstudio\lib\site-packages\vnpy\app\portfolio_strategy\strategies\pair_trading_strategy.py", line 107, in on_tick
self.on_bars(bars)
File "C:\vnstudio\lib\site-packages\vnpy\app\portfolio_strategy\strategies\pair_trading_strategy.py", line 127, in on_bars
leg1_bar.close_price self.leg1_ratio - leg2_bar.close_price self.leg2_ratio
AttributeError: 'NoneType' object has no attribute 'close_price'
请问如何解决？
