 
 
     close: =>
-        @execution_queue.close()
+        @execution_queue?.close()
         super()
 
     _apply_changeObj: (changeObj) =>

        @close_on_action()
        @clear_action_flags(true)
        # Empty the execution queue.
        @execution_queue.clear()
        @process_sage_updates(caller:"kill")
        if opts.restart
            @restart(cb:opts.cb)

            opts.cb?(); return
        @close_on_action()
        t = misc.walltime()
        @execution_queue?.clear()
        @clear_action_flags(false)
        async.series([
            (cb) =>
