Thanks, menotu.
I installed the extension pack and now I can once again save the guest machine status without it bombing on the next startup.
I did have a few moments of consternation while trying to follow your instructions because you do not mention that you have to find and click on a very tiny icon on the right hand side before you get anything about adding the extension.
This might be one of those setup things where you can have text showing with the icon. Mine shows no text unless I click on it.