为什么文字下面会有两条下划线呢？

来源：10-2 基于ListView实现水平和垂直方式滚动的列表【边学边做】

常师傅666

2020-04-20

import 'package:flutter/material.dart';

const CITY_NAME = ['北京','天津','上海','南京','上海','济南','青岛','北京','天津','上海','南京','上海','济南','青岛'];

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


class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: ListView(
        children: _buildList(),
      ),
    );
  }

  List<Widget>_buildList(){
    return CITY_NAME.map((city)=>_item(city)).toList();
  }
  Widget _item(city){
    return Container(
      height: 80,
      margin: EdgeInsets.only(bottom: 5),
      alignment: Alignment.center,
      decoration: BoxDecoration(color: Colors.teal),
      child: Text(city,
      style: TextStyle(color: Colors.white,fontSize: 20),),
    );
  }
}

图片描述
为什么文字下面会有两条下划线呢？