爬取失败，报错 AttributeError: 'NoneType' object has no attribute 'xpath'

[waynezhao12]

crawling : http://www.autolabor.com.cn/book/ROSTutorials/di-7-zhang-ji-qi-ren-dao-822a28-fang-771f29/104-ben-zhang-xiao-jie.html
Traceback (most recent call last):
File "/app/gitbook.py", line 303, in
Gitbook2PDF(url).run()
File "/app/gitbook.py", line 192, in run
loop.run_until_complete(self.crawl_main_content(content_urls))
File "/usr/local/lib/python3.7/asyncio/base_events.py", line 583, in run_until_complete
return future.result()
File "/app/gitbook.py", line 214, in crawl_main_content
await asyncio.gather(*tasks)
File "/app/gitbook.py", line 235, in gettext
text = ChapterParser(metatext, title, level, ).parser()
File "/app/gitbook.py", line 92, in parser
if tree.xpath('//section[@Class="normal markdown-section"]'):
AttributeError: 'NoneType' object has no attribute 'xpath'

请问如何解决？