[Bug] mkdocstrings unable to parse custom theme directory configuration

[abhiTronix]

Hello,

mkdocstrings is incompatible with mkdocs-material's custom_dir configuration:

theme:
  name: null
  custom_dir: mkdocs-material/material

and thereby fails with following error:

DEBUG   -  Reading: api.md 
DEBUG   -  mkdocstrings.extension: Matched '::: vidgear.gears' 
DEBUG   -  mkdocstrings.extension: Using handler 'python' 
DEBUG   -  mkdocstrings.handlers.python: Opening 'pytkdocs' subprocess 
ERROR   -  Error reading page 'api.md': expected str, bytes or os.PathLike object, not NoneType 
Traceback (most recent call last):
  File "/usr/local/bin/mkdocs", line 8, in <module>
    sys.exit(cli())
  File "/usr/local/lib/python3.6/dist-packages/click/core.py", line 829, in __call__
    return self.main(*args, **kwargs)
  File "/usr/local/lib/python3.6/dist-packages/click/core.py", line 782, in main
    rv = self.invoke(ctx)
  File "/usr/local/lib/python3.6/dist-packages/click/core.py", line 1259, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/local/lib/python3.6/dist-packages/click/core.py", line 1066, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/usr/local/lib/python3.6/dist-packages/click/core.py", line 610, in invoke
    return callback(*args, **kwargs)
  File "/usr/local/lib/python3.6/dist-packages/mkdocs/__main__.py", line 136, in serve_command
    **kwargs
  File "/usr/local/lib/python3.6/dist-packages/mkdocs/commands/serve.py", line 141, in serve
    config = builder()
  File "/usr/local/lib/python3.6/dist-packages/mkdocs/commands/serve.py", line 136, in builder
    build(config, live_server=live_server, dirty=dirty)
  File "/usr/local/lib/python3.6/dist-packages/mkdocs/commands/build.py", line 271, in build
    _populate_page(file.page, config, files, dirty)
  File "/usr/local/lib/python3.6/dist-packages/mkdocs/commands/build.py", line 171, in _populate_page
    page.render(config, files)
  File "/usr/local/lib/python3.6/dist-packages/mkdocs/structure/pages.py", line 175, in render
    self.content = md.convert(self.markdown)
  File "/usr/local/lib/python3.6/dist-packages/markdown/core.py", line 265, in convert
    root = self.parser.parseDocument(self.lines).getroot()
  File "/usr/local/lib/python3.6/dist-packages/markdown/blockparser.py", line 90, in parseDocument
    self.parseChunk(self.root, '\n'.join(lines))
  File "/usr/local/lib/python3.6/dist-packages/markdown/blockparser.py", line 105, in parseChunk
    self.parseBlocks(parent, text.split('\n\n'))
  File "/usr/local/lib/python3.6/dist-packages/markdown/blockparser.py", line 123, in parseBlocks
    if processor.run(parent, blocks) is not False:
  File "/usr/local/lib/python3.6/dist-packages/mkdocstrings/extension.py", line 140, in run
    **handler_config,
  File "/usr/local/lib/python3.6/dist-packages/mkdocstrings/handlers/__init__.py", line 233, in get_handler
    HANDLERS_CACHE[name] = module.get_handler(theme, custom_templates, **config)  # type: ignore
  File "/usr/local/lib/python3.6/dist-packages/mkdocstrings/handlers/python.py", line 249, in get_handler
    renderer=PythonRenderer("python", theme, custom_templates),
  File "/usr/local/lib/python3.6/dist-packages/mkdocstrings/handlers/__init__.py", line 118, in __init__
    paths.append(themes_dir / theme)
  File "/usr/lib/python3.6/pathlib.py", line 899, in __truediv__
    return self._make_child((key,))
  File "/usr/lib/python3.6/pathlib.py", line 686, in _make_child
    drv, root, parts = self._parse_args(args)
  File "/usr/lib/python3.6/pathlib.py", line 640, in _parse_args
    a = os.fspath(a)
TypeError: expected str, bytes or os.PathLike object, not NoneType