A warning wouldn’t be a problem if you weren’t using header redirects through php.
This warning causes my redirects not to work because the warning, once printed, counts as an already sent header.
The only solution I came up with was disabling the plugin.
Sad thing
It’s not essential, but if you have modified some core files (as you said) you will lose your edits when you will update your theme. In order to prevent this, just don’t update. However, you could have some security problems in the near (or far) future.
Quotating https://www.elegantthemes.com/blog/resources/wordpress-child-theme-tutorial, “According to our recent WordPress Themes Survey, 85% of our customers customize their themes, and only 35% use a child theme when doing so.”.
So don’t feel alone
