tab crash

import 'package:flutter/material.dart';

void main() => runApp(new TabBarDemo());

class TabBarDemo extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: DefaultTabController(
        length: 4,
        child: Scaffold(
          appBar: AppBar(
            bottom: TabBar(
              tabs: [
                Tab(text: "1"),
                Tab(text: "2"),
                Tab(text: "3"),
                Tab(text: "4"),
              ],
            ),
            title: Text('Select last tab'),
          ),
          body: TabBarView(
            children: [
              TabContent("1"),
              TabContent("2"),
              TabContent("3"),
              TabContent("4"),
            ],
          ),
        ),
      ),
    );
  }
}

class TabContent extends StatefulWidget {
  final String name;

  TabContent(this.name);

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

class _TabContentState extends State<TabContent>
    with AutomaticKeepAliveClientMixin<TabContent> {
  @override
  bool get wantKeepAlive => true;

  int count = 0;

  @override
  Widget build(BuildContext context) {
    return Center(
      child: Column(
        mainAxisAlignment: MainAxisAlignment.center,
        children: <Widget>[
          Text(widget.name),
          SizedBox(
            height: 80.0,
          ),
          Text(
            count.toString(),
            style: Theme.of(context).textTheme.display1,
          ),
          OutlineButton(
            child: Text("increment"),
            onPressed: _increment,
          ),
        ],
      ),
    );
  }

  void _increment() => setState(() {
        count++;
      });
}