private void detachController() {
    if (!mIsControllerAttached) {
      return;
    }
    mEventTracker.recordEvent(Event.ON_DETACH_CONTROLLER);
    mIsControllerAttached = false;
    if (isControllerValid()) {
      // NULLSAFE_FIXME[Nullable Dereference]
      mController.onDetach();
    }
  }
Returns whether currently set controller is valid: not null and attached to the hierarchy that
is held by the holder
drawee/src/main/java/com/facebook/drawee/view/DraweeHolder.java