It was a pain that the process would fail if you didn't name your render ahead of time, and it got a little confusing that it didn't attach the blend file name to the subject line unless you had saved it.
Both of these issues are addressed with this release. The addon now automatically names your render file as soon as Blender is opened, and sets it to be saved in your user's Documents folder. It will also call an unsaved blend file - 'untitled.blend', rather than simply leaving the space blank.
These are only defaults, you can change them to whatever you like and the addon will reflect your filenames and locations.
The only catch with this update is that, in order to save the images after rendering, you must make some change to your scene. If you open Blender and immediately hit render, the same error will occur, and the addon will be unable to find your render.
But, as long as you make any kind of change to your scene, move, scale, rotate an object, change a material value, start frame etc, that will cause the addon to update, and the render to save.
This is not a bug that is likely to affect many people, as most people don't just want to render a default cube only, but it might crop up if you're testing.