I do not have enough rights to check the installation. Will need admin rights. Meanwhile, the error occurs if id attribute is missing from register_sidebar() function. I checked the theme code and we are setting the id attribute. Can you pls disable all plugins once and see if the error goes away? The notice can get displayed even if one of the plugins loaded calls this function with wrong arguments.
If you provide me the admin rights, I can do a check again if the above does not help resolve the issue. Thanks.