title: Python 3.9 Errors with MKDocs Serve
body: <p>After upgrading to Python 3.9, i'm getting these errors when running <code>mkdocs serve</code> I'm not a developer. I use MKDocs to build a documentation site and I wanted to upgrade the software, as a result I had to upgrade Python, pip, mkdocs and mkdocs-material..</p>
  <pre><code>INFO    -  Cleaning site directory 
  Traceback (most recent call last):
    File &quot;/usr/local/bin/mkdocs&quot;, line 8, in &lt;module&gt;
      sys.exit(cli())
    File &quot;/usr/local/lib/python3.9/site-packages/click/core.py&quot;, line 829, in __call__
      return self.main(*args, **kwargs)
    File &quot;/usr/local/lib/python3.9/site-packages/click/core.py&quot;, line 782, in main
      rv = self.invoke(ctx)
    File &quot;/usr/local/lib/python3.9/site-packages/click/core.py&quot;, line 1259, in invoke
      return _process_result(sub_ctx.command.invoke(sub_ctx))
    File &quot;/usr/local/lib/python3.9/site-packages/click/core.py&quot;, line 1066, in invoke
      return ctx.invoke(self.callback, **ctx.params)
    File &quot;/usr/local/lib/python3.9/site-packages/click/core.py&quot;, line 610, in invoke
      return callback(*args, **kwargs)
    File &quot;/usr/local/lib/python3.9/site-packages/mkdocs/__main__.py&quot;, line 133, in serve_command
      serve.serve(
    File &quot;/usr/local/lib/python3.9/site-packages/mkdocs/commands/serve.py&quot;, line 141, in serve
      config = builder()
    File &quot;/usr/local/lib/python3.9/site-packages/mkdocs/commands/serve.py&quot;, line 136, in builder
      build(config, live_server=live_server, dirty=dirty)
    File &quot;/usr/local/lib/python3.9/site-packages/mkdocs/commands/build.py&quot;, line 263, in build
      nav = get_navigation(files, config)
    File &quot;/usr/local/lib/python3.9/site-packages/mkdocs/structure/nav.py&quot;, line 102, in get_navigation
      items = _data_to_navigation(nav_config, files, config)
    File &quot;/usr/local/lib/python3.9/site-packages/mkdocs/structure/nav.py&quot;, line 157, in _data_to_navigation
      return [
    File &quot;/usr/local/lib/python3.9/site-packages/mkdocs/structure/nav.py&quot;, line 158, in &lt;listcomp&gt;
      _data_to_navigation(item, files, config)[0]
    File &quot;/usr/local/lib/python3.9/site-packages/mkdocs/structure/nav.py&quot;, line 150, in _data_to_navigation
      return [
    File &quot;/usr/local/lib/python3.9/site-packages/mkdocs/structure/nav.py&quot;, line 153, in &lt;listcomp&gt;
      Section(title=key, children=_data_to_navigation(value, files, config))
    File &quot;/usr/local/lib/python3.9/site-packages/mkdocs/structure/nav.py&quot;, line 164, in _data_to_navigation
      file = files.get_file_from_path(path)
    File &quot;/usr/local/lib/python3.9/site-packages/mkdocs/structure/files.py&quot;, line 31, in get_file_from_path
      return self.src_paths.get(os.path.normpath(path))
    File &quot;/usr/local/Cellar/python@3.9/3.9.0_1/Frameworks/Python.framework/Versions/3.9/lib/python3.9/posixpath.py&quot;, line 336, in normpath
      path = os.fspath(path)
  TypeError: expected str, bytes or os.PathLike object, not NoneType```
  </code></pre>