我用硒测试写django网站。我在django和selenium docs的测试中编写了selenium的设置代码。每次测试后，我都会在控制台上看到警告。
Traceback (most recent call last):
File "/usr/lib/python3.5/wsgiref/handlers.py", line 137, in run
self.result = application(self.environ, self.start_response)
File "/home/android/.virtenvs/webdjango/lib/python3.5/site-packages/django/test/testcases.py", line 1193, in __call__
return super(FSFilesHandler, self).__call__(environ, start_response)
File "/home/android/.virtenvs/webdjango/lib/python3.5/site-packages/django/core/handlers/wsgi.py", line 177, in __call__
response = self.get_response(request)
File "/home/android/.virtenvs/webdjango/lib/python3.5/site-packages/django/test/testcases.py", line 1176, in get_response
return self.serve(request)
File "/home/android/.virtenvs/webdjango/lib/python3.5/site-packages/django/test/testcases.py", line 1188, in serve
return serve(request, final_rel_path, document_root=self.get_base_dir())
File "/home/android/.virtenvs/webdjango/lib/python3.5/site-packages/django/views/static.py", line 54, in serve
fullpath = os.path.join(document_root, newpath)
File "/home/android/.virtenvs/webdjango/lib/python3.5/posixpath.py", line 89, in join
genericpath._check_arg_types('join', a, *p)
File "/home/android/.virtenvs/webdjango/lib/python3.5/genericpath.py", line 143, in _check_arg_types
(funcname, s.__class__.__name__)) from None
TypeError: join() argument must be str or bytes, not 'NoneType'