void dispose()

Remove this entry from the overlay and restore the widget to its original place in the tree.

Once removed, the overlay entry cannot be used further.

Source 

void dispose() {
  _targetKey = null;
  _curve = null;
  _controller?.dispose();
  _controller = null;
  _handle.stopMimic();
  _handle = null;
  _overlayEntry.remove();
  _overlayEntry = null;
}