That is the old user interface file that isn't used in Design Extender any more.
What that means is that you have another extension installed that is trying to use that file and uninstalling the other extensions removed it.
I'll attach a copy of that file to this thread and you can add it back in. If other files were removed it might mean you have to reinstall one of the other webassist extensions you have which is the one that is actually throwing this error.