Thanks for writing in! Upon testing your referenced site, some times I received the following error when loading Cornerstone (see secure note).
Could you please try increasing your PHP Memory limit by editing your wp-config.php file which can be located under the root of your WordPress installation. Add the following lines into that file.
define( 'WP_MEMORY_LIMIT', '512M' );
define( 'WP_MAX_MEMORY_LIMIT', '512M' );
You can add it before the line
/*That's all, stop editing! Happy Blogging. */
If not, please follow our troubleshooting guide here (https://theme.co/apex/forum/t/troubleshooting-builder-preview-warnings/14404).