Nullify view controllers
Nullify view controllers

Incoming Call View Controller, Active Call View Controller, and Ended
Call View Controller have bindings in the UI to Call Controller's
properties. When Call Controller is allocated on call window close,
those view controller's still doing their KVO and an error is
issued. By this nullifying we release view controllers before Call
Controller is released.

mralexgray/SwitchBoard,inz/Telephone,nriley/Telephone,einfallstoll/Telephone,thierryc/Telephone,mobdim/Telephone