Showing ToggleButtons does not remove border

main.dart

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Toggle Button',
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Toggle Button'),
    );
  }
}

class MyHomePage extends StatelessWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(title),
      ),
      body: Center(
        child: ChooseIcon(),  
      ),
    );
  }
}

class ChooseIcon extends StatefulWidget {

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

class ChooseIconState extends State<ChooseIcon> {

  final icons = [
    Icon(Icons.ac_unit),
    Icon(Icons.call),
    Icon(Icons.cake),
  ];

  List<bool> _selection;

  @override
  void initState() {
    super.initState();
    _selection = List.filled(icons.length, false);
  }

  @override
  Widget build(BuildContext context) {
    return Row(
        children: [
          Spacer(),
          ToggleButtons(
            borderWidth: null, // should remove border
            children: icons,
            isSelected: _selection,
            onPressed: (int index) {
              setState(() {
                for (int buttonIndex = 0; buttonIndex < _selection.length; buttonIndex++) {
                  if (buttonIndex == index) {
                    _selection[buttonIndex] = !_selection[buttonIndex];
                  } else {
                    _selection[buttonIndex] = false;
                  }
                }
              });
            },
          ),
          Spacer(),
        ],
      );
  }
}