@Hackwar can you have a lock at this please?

Same error if set Mail Format to "HTML".

The reason that there no HTML template, for the test message, for com_config.test_mail.
Same error will be for all emails without HTML template, in administrator/index.php?option=com_mails&view=templates,
And vice versa, when Mail Format to "plaintext" but email template for "plaintext" is empty.

I guess we need some kind of fallback in Joomla\CMS\Mail\MailTemplate, something to:

// Html fallback from plaintext
if (!$mail->htmlbody && $mail->body)
{
  $mail->htmlbody = nl2br($mail->body)
}

// Plaintext fallback from HTML
if (!$mail->body && $mail->htmlbody)
{
  $mail->body = strip_tags(str_replace(['<br>', '<br />'], "\n", $mail->htmlbody));
}

This will work if one of template is missing, and fail when both templates is empty (that is okay).

@Hackwar what do you think? I can try to make a PR

@Fedik If you have the time, please be my guest. It would be greatly appreciated. I'm currently struggling to get stuff like this done.

please test #32118