You are getting the error because memento has been removed from our extensions. Dreamweaver keeps track of what windows were open the last time it was closed and tries to re-open any windows in the position they last appeared when you re-open Dreamweaver.
The error is telling you that memento no longer exists so it can't be positioned.
In order to get rid of the error you can go to: window->workspace layout->reset 'your workspace'
This will usually work unless you have defined a new custom workspace and you had memento open when you did. In that case you would have to reset your workspace to one of the dreamweaver defaults and recreate and re-save the workspace layout you want to use.