section-index not working with material from mkdocs 9.5.19+

[tuliptripper]

First reported here https://github.com/squidfunk/mkdocs-material/discussions/7225. They suggested opening an issue here.

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I'm trying to upgrade to mkdocs-material 9.5.19, but my builds are failing. I'm not a python expert, and I'm wondering if anyone has some insight into how to fix this. I believe that it might have something to do with with the mkdocs-section-index plugin. I'm using the latest version of that (0.3.9).

Here is the error I'm getting:

Traceback (most recent call last):
  File "/usr/local/bin/mkdocs", line 8, in <module>
    sys.exit(cli())
             ^^^^^
  File "/usr/local/lib/python3.11/site-packages/click/core.py", line 1157, in __call__
    return self.main(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/click/core.py", line 1078, in main
    rv = self.invoke(ctx)
         ^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/click/core.py", line 1688, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/click/core.py", line 1434, in invoke
    return ctx.invoke(self.callback, **ctx.params)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/click/core.py", line 783, in invoke
    return __callback(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/mkdocs/__main__.py", line 284, in build_command
    build.build(cfg, dirty=not clean)
  File "/usr/local/lib/python3.11/site-packages/mkdocs/commands/build.py", line 299, in build
    nav = config.plugins.on_nav(nav, config=config, files=files)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/mkdocs/plugins.py", line 596, in on_nav
    return self.run_event('nav', nav, config=config, files=files)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/mkdocs/plugins.py", line 566, in run_event
    result = method(item, **kwargs)
             ^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/mkdocs_section_index/plugin.py", line 33, in on_nav
    assert not page.children
           ^^^^^^^^^^^^^^^^^
AssertionError

Any insight or assistance would be greatly appreciated.