I was getting the same error, and I could not find the file to copy that others had mentioned. However, I found a fix that works for me. I did a "control + click" on the facetime icon in the dock, selected "options" then "remove from dock". Once it was gone, I opened "launchpad" opened Facetime from launchpad, then when the icon was in the dock I did another "control + click", "options" and then "keep in dock". Now I have no problem opening it. Hope this helps someone else.