Receiving error message when editing theme

I’m receiving the following error message when i try to update files in the theme editor(Edit: Or when I try to upload images, publish posts, etc.):

Warning: Cannot modify header information – headers already sent by (output started at C:\xampp\htdocs\wordpress\wp-includes\formatting.php:1) in C:\xampp\htdocs\wordpress\wp-admin\theme-editor.php on line 69

Whereas line 69 consists of:

header(“Location: $location”);

The files are still uppdated as usually, but I’d prefer not to have the error message appear. Does anyone have any idea about what could cause the error, and how I can fix it?

Thanks!