Wrestling with an error message on a Drupal Commerce site today. I kept getting errors relating to a missing template file with no path and no file name .tpl.php. Fortunatley with a little debugging giving me a pointer to the correct issue queue on Drupal the solution was readily available. So here's a few easy steps to follow in case you have the same issue.

Initially thought it was to do with Address Field formatting with a PDF invoice module - but only because that was the last update to the site and the error appeared when I returned to the Order list screen.

Debugging the code showed that that was a red herring and theme.inc highlighted the Order screen to be the culprit.
What appears to be happening is that a theme hook is giving a suggestion pattern for a template but not completing the job - and therefore we are left with no path, no file name and just a template extension - thus htdoc/.tpl.php

With a little further delving and knowing that it was the Order screen that was causing the problem - I found a solution here

Basically it is due to Display Suite not getting a default layout for the order display. This may happen under other cirumstances and therefore worth noting! - Also the fix shouldn't be necessary and there may be a fix from one of the modules in the future.

Steps to fix:

Step one: Create a layout for display suite in the Order setting display (admin/commerce/config/order/display)

Choose One Column as that won't affect your actual layout.

Step Two: Note that the Fields are all now disabled - Set them to display in the Content region

Step Three: That's it - now you will have no more of those errors (for the Order screen....)

Thanks again to gagoo for the solution!