Same here. Had run into an editing issue in /libraries/cms/application/site.php
Found: forum.joomla.org/viewtopic.php?f=709&t=825504

and changed $lang_code = $this->getLanguage()->getTag();
to $lang_code = JFactory::getLanguage()->getTag();
to get rid of a fatal error.
That worked (I think) until just upgraded the JoomSEF to 4.6.4.

Now getting that fatal error: Fatal error: Using $this when not in object context in /components/com_sef/joomsef.php on line 644

It seems to go away if you disable Auto Update URLs as mentioned in the forum thread. But that isn't necessarily good. Help?

Got the message Warning: Creating default object from empty value in /admin/helpers/artio-update.php on line 94, when trying to update JoomSEF from 4.5.2 to 4.6.2.

This sounds like it could be a bug, but is it anything I should be concerned about?

Thanks!