2-level tab bar error minimal case

main.dart

import 'package:flutter/material.dart';

void main() {
  runApp(
    new MaterialApp(
      title: 'Flutter Demo',
      theme: new ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: new FlutterDemo(),
    ),
  );
}

class FlutterDemo extends StatefulWidget {
  FlutterDemo({Key key}) : super(key: key);

  @override
  _FlutterDemoState createState() => new _FlutterDemoState();
}

class _FlutterDemoState extends State<FlutterDemo> {
  Map<int, Color> _activeColors = Colors.red;

  Widget _makeTabBar(BuildContext context) {
    final steps = [200, 300, 400, 500, 600, 700];
    final colorSteps = steps.map((step) => _activeColors[step]).toList();
    final colorWidgets = colorSteps.map((color) {
      return new Container(
          width: 50.0,
          height: 50.0,
          decoration: new BoxDecoration(backgroundColor: color));
    }).toList(growable: false);
    final tbs = new TabBarSelection<Color>(
        child: new TabBarView<Color>(children: colorWidgets),
        values: colorSteps,
        value: colorSteps[0]);
    return tbs;
  }

  _red() {
    setState(() {
      _activeColors = Colors.red;
    });
  }

  _green() {
    setState(() {
      _activeColors = Colors.green;
    });
  }

  _blue() {
    setState(() {
      _activeColors = Colors.blue;
    });
  }

  Widget _makeButtons() {
    final red = new FlatButton(child: new Text('red'), onPressed: _red);
    final green = new FlatButton(child: new Text('green'), onPressed: _green);
    final blue = new FlatButton(child: new Text('blue'), onPressed: _blue);
    return new ButtonBar(children: [red, green, blue]);
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text('Flutter Demo'),
      ),
      body: new Column(children: [
        _makeButtons(),
        new Container(height: 200.0, child: _makeTabBar(context))
      ]),
    );
  }
}