FlutterSlider doesn't work as a persistenbutton in the Scaffold Widget

[xcarol]

Just adding the Single slider example shown in the Readme page as a persistentFooterButtons into the Scaffold Widget

persistentFooterButtons: [
        FlutterSlider(
          values: [300],
          max: 500,
          min: 0,
          onDragging: (handlerIndex, lowerValue, upperValue) {
            _lowerValue = lowerValue;
            _upperValue = upperValue;
            setState(() {});
          },
        ),
      ],

in the example provided to see these logs:

  size: MISSING
This RenderObject had the following descendants (showing up to depth 5):
    child: RenderPadding#273ee NEEDS-LAYOUT NEEDS-PAINT NEEDS-COMPOSITING-BITS-UPDATE
      child: RenderPositionedBox#39e03 NEEDS-LAYOUT NEEDS-PAINT NEEDS-COMPOSITING-BITS-UPDATE
        child: _RenderOverflowBar#32333 NEEDS-LAYOUT NEEDS-PAINT NEEDS-COMPOSITING-BITS-UPDATE
          child 1: _RenderLayoutBuilder#419c0 NEEDS-LAYOUT NEEDS-PAINT
════════════════════════════════════════════════════════════════════════════════════════════════════
5
Another exception was thrown: Assertion failed: file:///home/xcarol/snap/flutter/common/flutter/packages/flutter/lib/src/rendering/box.dart:1972:12
9
Another exception was thrown: Unexpected null value.
20
Another exception was thrown: Cannot hit test a render box that has never been laid out.

in the debug console. Nothing appears in the browser but similar logs to the above in the browser console.