Have you checked if Version Selector is really missing, or if Mendix is just unable to find it? By default, it would be in “C:\Program Files\Mendix\Version Selector”. If you can find it there, try selecting the .exe when the pop-up opens up, and see if it works next time.
I have uninstalled all versions of Mx 8, including the beta releases, and then reinstalled. Then the version selector was found.