Steps to reproduce the issue

Try to send mail using a mail template which does not exist.

$mailer = new MailTemplate('com_bftest.invalidtemplateid', $language);
$mailer->addRecipient($templateData['emailto'], $templateData['emailtoname']);
$mailer->setReplyTo($templateData['replytoemail'], $templateData['replytoname']);
$mailer->addTemplateData($templateData);
$sent = $mailer->send();

Expected result

Sensible error message saying template does not exist

Actual result

Load of PHP notices and a 'Message body empty' warning.

System information (as much as possible)

Additional comments

Be better to add a getInstance() static method to MailTemplate which would allow this

$mailer = MailTemplate::getInstance('com_bftest.invalidtemplateid', $language);
if (empty($mailer))
{
     ... take some action ...
}
$mailer->addRecipient($templateData['emailto'], $templateData['emailtoname']);
$mailer->setReplyTo($templateData['replytoemail'], $templateData['replytoname']);
$mailer->addTemplateData($templateData);
$sent = $mailer->send();

Can do this as a workaround - downside MailTemplate::getTemplate() gets called twice.

$test = MailTemplate::getTemplate($templateId, $language);
if (empty($mail))
{
     ... take some action ...
}
$mailer = new MailTemplate('com_bftest.invalidtemplateid', $language);
$mailer->addRecipient($templateData['emailto'], $templateData['emailtoname']);
$mailer->setReplyTo($templateData['replytoemail'], $templateData['replytoname']);
$mailer->addTemplateData($templateData);
$sent = $mailer->send();