plugin still works, but throws an error

as of 4.7 the plugin works as described, but if you enable debug this error shows up since some versions ago on some pages in the admin

Strict Standards: call_user_func_array() expects parameter 1 to be a valid callback, non-static method tfnab_smart_quotes::admin_init() should not be called statically in /home/public_html/wp-includes/class-wp-hook.php on line 298

thought you might want to know