Draggable widget fails on Web

main.dart

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  final overlayCtnKey = GlobalKey();

  @override
  Widget build(BuildContext context) {
    final fab = FloatingActionButton(
        onPressed: () {}); // works if switch to a widget without onTap/onPress...

    return MaterialApp(
      home: Stack(
        children: [
          Container(key: overlayCtnKey),
          DraggableWidget(containerKey: overlayCtnKey, child: fab),
        ],
      ),
    );
  }
}

class DraggableWidget extends StatefulWidget {
  final Widget child;

  final GlobalKey containerKey;

  DraggableWidget({@required this.child, @required this.containerKey});

  @override
  DraggableWidgetState createState() => DraggableWidgetState();
}

class DraggableWidgetState extends State<DraggableWidget> {
  double top, left;

  @override
  void initState() {
    top = 50;
    left = 50;
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    final RenderBox backgroundRenderBox = widget.containerKey.currentContext.findRenderObject();
    return Positioned(
      top: top,
      left: left,
      child: Draggable(
        dragAnchor: DragAnchor.child,
        feedback: widget.child,
        child: widget.child,
        childWhenDragging: const SizedBox(),
        onDragEnd: (drag) {
          final localOffset = backgroundRenderBox.globalToLocal(drag.offset);
          setState(() {
            top = localOffset.dy;
            left = localOffset.dx;
          });
        },
      ),
    );
  }
}