Can't Find that PHP Error in My WP Theme, Which the reviewer stated.

I Submitted my theme to Themeforest (Premium Marketplace), The reviewed my theme and figured out that the theme has this error :

Warning: wp-admin/admin-header.php:9 Cannot modify header information - headers already sent by (output started at /home/wp- includes/function.php:3894) require_once(‘wp-admin/admin- header.php’), header

But whenever I check the theme I can’t find this error, I also tried checking the theme with wp_debug set “true”, where can find this error in my theme? & how to solve this error.

This is what the reviewer stated :

Please make sure that your theme doesn’t raise any PHP errors, notices or warnings. Enabling wp_debug can help you with this.

Headers already sent is very common PHP error (you should see it if you have WP_DEBUG enabled in “wp-config.php”), sometimes tricky to find. Also XDebug ( ) installed on your server would help a lot
Here’s one link that might help you, but if not, google for “headers already sent”: