ビルドが通らない（mkdocs-with-pdf）

実現したいこと

MkDocsで作成したものをPFDで書き出したい。
「mkdocs-with-pdf」を使って書き出そうとしていたところ、
ビルドが通らなくなってしまい、困っています。

▼バージョン
python3
-Python 3.9.13

pip3
-pip 22.1.2 from /home/linuxbrew/.linuxbrew/opt/python@3.9/lib/python3.9/site-packages/pip (python 3.9)

mkdocs
-mkdocs, version 1.3.0 from /home/linuxbrew/.linuxbrew/opt/python@3.9/lib/python3.9/site-packages/mkdocs (Python 3.9)

brew
-Homebrew 3.5.4
-Homebrew/homebrew-core (git revision 83c40cc522a; last commit 2022-07-11)

発生している問題・エラーメッセージ

userT:/mnt/c/Project/mk_manual$ mkdocs build
INFO     -  Cleaning site directory
INFO     -  Building documentation to directory: /mnt/c/Project/mk_manual/site
INFO     -  Number headings up to level 3.
INFO     -  Generate a table of contents up to heading level 2.
Traceback (most recent call last):
  File "/home/linuxbrew/.linuxbrew/bin/mkdocs", line 8, in <module>
    sys.exit(cli())
  File "/home/linuxbrew/.linuxbrew/opt/python@3.9/lib/python3.9/site-packages/click/core.py", line 1130, in __call__
    return self.main(*args, **kwargs)
  File "/home/linuxbrew/.linuxbrew/opt/python@3.9/lib/python3.9/site-packages/click/core.py", line 1055, in main
    rv = self.invoke(ctx)
  File "/home/linuxbrew/.linuxbrew/opt/python@3.9/lib/python3.9/site-packages/click/core.py", line 1657, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/linuxbrew/.linuxbrew/opt/python@3.9/lib/python3.9/site-packages/click/core.py", line 1404, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/linuxbrew/.linuxbrew/opt/python@3.9/lib/python3.9/site-packages/click/core.py", line 760, in invoke
    return __callback(*args, **kwargs)
  File "/home/linuxbrew/.linuxbrew/opt/python@3.9/lib/python3.9/site-packages/mkdocs/__main__.py", line 192, in build_command
    build.build(config.load_config(**kwargs), dirty=not clean)
  File "/home/linuxbrew/.linuxbrew/opt/python@3.9/lib/python3.9/site-packages/mkdocs/commands/build.py", line 317, in build
    config['plugins'].run_event('post_build', config=config)
  File "/home/linuxbrew/.linuxbrew/opt/python@3.9/lib/python3.9/site-packages/mkdocs/plugins.py", line 104, in run_event
    result = method(**kwargs)
  File "/home/linuxbrew/.linuxbrew/opt/python@3.9/lib/python3.9/site-packages/mkdocs_with_pdf/plugin.py", line 135, in on_post_build
    self.generator.on_post_build(config, self.config['output_path'])
  File "/home/linuxbrew/.linuxbrew/opt/python@3.9/lib/python3.9/site-packages/mkdocs_with_pdf/generator.py", line 130, in on_post_build
    make_indexes(soup, self._options)
  File "/home/linuxbrew/.linuxbrew/opt/python@3.9/lib/python3.9/site-packages/mkdocs_with_pdf/toc.py", line 66, in make_indexes
    h1li.append(h2ul)
AttributeError: 'NoneType' object has no attribute 'append'