Thanks for sending me this login info. I can see that it was not my plugin that caused this error on your site.
The errors are caused be the static calls to get_sidebars() and name_to_class() on lines 52 and 57 in wp-content/themes/AVADA-3.0/framework/plugins/multiple_sidebars.php
My plugin didn’t touch that file and I restored the two files it did fix in your theme but that did not fix the error so I removed those two malicious threats again. It looks like that theme is using bad code and it needs to be fixed. Maybe there is an update or something. You should check with the developer or reinstall the theme (or another theme).