I've got the same problem; except I've already planted the fourth core...
My journal updated, saying I've killed Visas' master (Darth Nihilus) and planted the cores; all that's left is to escape. But that's the problem, I cannot leave! I ran back to the entrance, and walk up to the door, but nothing happens. I am stuck on the Ravager. I have wandered around the place numerous times, but I just cannot get off... Any ideas?