We just encountered the same Error after upgrading to OJS 3.1.2. An editor was not able to publish an article in the pipeline for production. On the Server the logs showed the same PHP Fatal error: Uncaught Error: Call to a member function getUserVar() on null in /var/www/html/plugins/generic/orcidProfile/OrcidProfilePlugin.inc.php:679 when I tried the same (last) step in the publication process.
Thanks to @Vitaliy mentioning the issue I updated the OrcidProfilePlugin (which is not in use!) to Orcid Profile v1.1.1-1 and after that the publishing process went through and worked.
As mentioned above the strange thing is that the plugin is not activated and stalls the publication.
