出现问题日志:
22:10:26 IF88.CFFEX-1m开始下载历史数据
22:10:26 数据下载失败，触发异常：
Traceback (most recent call last):
File "C:\vnstudio\lib\site-packages\vnpy\app\cta_backtester\engine.py", line 382, in run_downloading
data = rqdata_client.query_history(req)
File "C:\vnstudio\lib\site-packages\vnpy\trader\rqdata.py", line 142, in query_history
if rq_symbol not in self.symbols:
TypeError: argument of type 'NoneType' is not iterable