handle property Null safety
The object that specifies how wide to make the gap injected by this render object.
This should be a handle owned by a RenderSliverOverlapAbsorber and a RenderNestedScrollViewViewport.
Implementation
SliverOverlapAbsorberHandle get handle => _handle;Implementation
set handle(SliverOverlapAbsorberHandle value) {
  assert(value != null);
  if (handle == value) {
    return;
  }
  if (attached) {
    handle.removeListener(markNeedsLayout);
  }
  _handle = value;
  if (attached) {
    handle.addListener(markNeedsLayout);
    if (handle.layoutExtent != _currentLayoutExtent ||
        handle.scrollExtent != _currentMaxExtent) {
      markNeedsLayout();
    }
  }
}