anchor.diff:  
     /// Computes the accumulated clipping bounds, in global coordinates.
     Rect _computeClipRect() {
  -    assert(RendererBinding.instance.renderView != null);
  -    var clipRect = Offset.zero & RendererBinding.instance.renderView.size;
  +    assert(RendererBinding.instance?.renderView != null);
  +    var clipRect = Offset.zero & RendererBinding.instance!.renderView.size;
   
       var parentLayer = parent;
       while (parentLayer != null) {
  
positive.code:     // of `addPostFrameCallback` or `scheduleFrameCallback` so that work will
      // be done even if a new frame isn't scheduled and without unnecessarily
      // scheduling a new frame.
      SchedulerBinding.instance
          .scheduleTask<void>(_processCallbacks, Priority.touch);
    }
  
  
negative.code:       if (isFirstUpdate) {
          // We're about to render a frame, so a post-frame callback is guaranteed
          // to fire and will give us the better immediacy than `scheduleTask<T>`.
          SchedulerBinding.instance!.addPostFrameCallback((timeStamp) {
            _processCallbacks();
          });
        }
  