课时3的策略开发，导入策略运营回测爆出如下错误
def on_bar(self, bar: BarData):
am = self.am
AttributeError: 'DemoStrategy' object has no attribute 'am'

但是在前面有根据课程有这个设置
def init(
self,
cta_engine: Any,
strategy_name: str,
vt_symbol: str,
setting: dict,
):

    super().__init__(cta_engine,strategy_name,vt_symbol,setting)    

    self.bg = BarGenerator(self.on_bar) 
    self.am = ArrayManager()  

是哪里的问题？