That error implies that you have uninstalled a webassist extension and then tried to open up Universal Email without a page open. I think if you have an open page when opening the interface you shouldn't get this error.
It looks like you have been struggling with this for a few days. If you post a U.S. phone number or Skype contact information if you are international, then I'll call you and do screen sharing so I can debug your problems and make sure you get up and running.