Error on mkdocs build command

[thebigmunch]

Traceback (most recent call last):
  File "c:\python38\lib\runpy.py", line 193, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "c:\python38\lib\runpy.py", line 86, in _run_code
    exec(code, run_globals)
  File "C:\Python38\Scripts\mkdocs.exe\__main__.py", line 7, in <module>
  File "c:\python38\lib\site-packages\click\core.py", line 829, in __call__
    return self.main(*args, **kwargs)
  File "c:\python38\lib\site-packages\click\core.py", line 782, in main
    rv = self.invoke(ctx)
  File "c:\python38\lib\site-packages\click\core.py", line 1259, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "c:\python38\lib\site-packages\click\core.py", line 1066, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "c:\python38\lib\site-packages\click\core.py", line 610, in invoke
    return callback(*args, **kwargs)
  File "c:\python38\lib\site-packages\mkdocs\__main__.py", line 159, in build_command
    build.build(config.load_config(**kwargs), dirty=not clean)
  File "c:\python38\lib\site-packages\mkdocs\commands\build.py", line 274, in build
    _populate_page(file.page, config, files, dirty)
  File "c:\python38\lib\site-packages\mkdocs\commands\build.py", line 177, in _populate_page
    page.content = config['plugins'].run_event(
  File "c:\python38\lib\site-packages\mkdocs\plugins.py", line 94, in run_event
    result = method(item, **kwargs)
  File "c:\python38\lib\site-packages\mkdocstrings\plugin.py", line 160, in on_page_content
    self.map_urls(page.canonical_url, item)
  File "c:\python38\lib\site-packages\mkdocstrings\plugin.py", line 173, in map_urls
    self.url_map[anchor.id] = base_url + anchor.url
TypeError: unsupported operand type(s) for +: 'NoneType' and 'str'

The error doesn't happen with mkdocs serve.

[pawamoy]

It's probably because you didn't define your site_url in mkdocs.yml.

I didn't find a better way than using page.canonical_url (which is None if site_url isn't defined) to build the links for cross-references that works both while serving and building.

I guess we should at least catch the exception to issue an error.