Warning appears in page editor after activation

Just installed this plugin to use with a custom child theme of twenty-nineteen on WP 5.3. When opening a page to edit, getting this warning message:

<b>Warning</b>: Cannot modify header information – headers already sent by (output started at ./wp.localhost/wp-includes/functions.php:1779) in <b>./wp.localhost/wp-admin/admin-header.php</b> on line <b>9</b>

I am aware you currently do not fully support 5.3 but just wanted to make sure it was on your radar. Seems like the rest of the basic functionality is working as expected.

Cheers and thanks for a great plugin!