Hello,
I upgraded from 2.4.2 to 3.0.0-beta 2.
I did this by using this plugin: https://wordpress.org/plugins/easy-theme-and-plugin-upgrades/
When the upgrade process was completed, it gave this notice:
Notice: Undefined index: themes in /nas/content/live/kfw/wp-content/mu-plugins/wpengine-security-auditor.php on line 200
The error is on the WP-Engine Security Auditor, which is a must-use plugin on WP-Engine installations.
The site works 100% fine as far as I can tell.
Just an FYI. Thanks!