message: prevent homing of nonexistent pip
diff: @@ -479,6 +479,7 @@ class Robot(object):
  target = self.pose_tracker[placeable].position + offset.coordinates
  other_instrument = {instrument} ^ set(self._instruments.values())
  + if not len(other_instrument) == 0:
  other_instrument.pop().mount_obj.home()
  if strategy == 'arc':
  