fromRoute used where fromUri should be

Trying to build a sitemap was crashing with this error:

Uncaught PHP Exception Symfony\\Component\\Routing\\Exception\\RouteNotFoundException: "Route "http://michelle.dev/d8b7/" does not exist." at /var/www/html/d8b7/core/lib/Drupal/Core/Routing/RouteProvider.php line 145, referer: http://michelle.dev/d8b7/batch?id=18&op=start

I finally tracked it down to line 244 of XmlSitemapGenerator.php where it was trying to use fromRoute on the base_url which was an actual URL, not a route. I changed it to fromUri and it's now generating with no errors.

Will attach a patch in a comment so I can get a node ID for it.