I got the same error with the the7 theme. Page would load but would get the error trying to get in the backend or going to a link. Used CPANEL to get to the File Manager or you could use FTP and just renamed theme file to disable it.
public_html/wp-content/themes/the7temp
The I was able to login in the backend and switched the theme. Updated by Duplicator Pro Plugin and had SG-OPTIMIZER but deactivated that also.. Doubt that had anything to do with it but I reactivated my 7 THEME and everything worked as it should.. Do not know what the issue was but everything worked after I did the steps above..
