Bug fix for “Message body empty”

This bug is partially resolved by https://github.com/timersys/wordpress-email-templates/commit/0b7b0ddee933403ccd1012925147b0fe53383e58.

However, for those that have implemented custom templates using this official guide, then the changes from the above commit need to be in your custom template files, too.

Anywhere you have include_once needs to be include to allow multiple includes (which is why you see error “Message body empty”; the template is only included once).
The “bug” was introduced in this PR which is resolved by the commit I linked above.
The PR should not be reverted as it correctly allows users to send multiple different templates in one request, so it’s a good feature. Just means others need to update their code like in the commit linked above.