请问为什么运行下载代码的程序时，会出现这样的情况呢：

Traceback (most recent call last):

  File "C:\Users\yuanqi\Desktop\dangdang.py", line 102, in <module>

    main()

  File "C:\Users\yuanqi\Desktop\dangdang.py", line 97, in main

    result = d.parse_item(items=items)

  File "C:\Users\yuanqi\Desktop\dangdang.py", line 50, in parse_item

    for item in items:

TypeError: 'NoneType' object is not iterable