I have the same warning with error.php of another template which uses T3 framework ( v 2.7.5 ) on PHP 7.2.15
The related line is " if($theme && is_file(T3_TEMPLATE_PATH ..."
I see the warning only in server's log so it is not actually a problem but i'm afraid that as soon as i will upgrade my PHP version this will throw an error instead of a warning.
Maybe a fix can be found defining the T3_TEMPLATE_PATH in a different way, like it was suggested in this thread : https://www.joomlart.com/forums/topic/joomlart-templates-dont-work-with-hhvm/ for the T3_TEMPLATE constant ?