I had the same problem.
My previous wordpress version is 4.9.5, and updating my wordpress(4.9.6) fixed this issue.
What is your wordpress version?
(P.S. my theme version-6.1.5 / cornerstone version - 3.1.5)
Those are notices that can be ignored. To turn off those notice messages, please set the WP_DEBUG from true to false under wp_config.php file locates in your root directory.
define('WP_DEBUG', false);
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