I had the same thing happen and when I asked the person who's extension was "Null", they had changed their status from "Available" to "Other" while not taking calls. When you click "other" you get a dialogue box that asks you to enter a value, not knowing what to put in the box if you hit cancel, the "Null" mouse over will result and the icon will change to a green box of the person trying to change their status. Change the status back to available and the green null icon will go away.
