你可以复制粘贴下面运行完整的代码
检查data != null，当数据为null时返回CircularProgressIndicator()
代码片段

body: data != null
            ? ListView.builder(itemBuilder: (context, index) {
                return ListTile(
                  title: Text(data[index]["title"]),
                );
              })
            : Center(child: CircularProgressIndicator()))

工作演示

完整代码

import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;
import 'dart:convert';

class Homepage extends StatefulWidget {
  @override
  _HomepageState createState() => _HomepageState();
}

class _HomepageState extends State<Homepage> {
  var url = "http://jsonplaceholder.typicode.com/photos";
  var data;
  // Used to initialize something before starting of the screen
  @override
  void initState() {
    // TODO: implement initState
    super.initState();
    fetchdata();
  }

  fetchdata() async {
    var res = await http.get(url);
    // print(res.body);

    // json parsing
    data = jsonDecode(res.body);
    print(data);

    //To tell the UI that we got the data
    setState(() {});
  }

  @override
  Widget build(BuildContext context) {
    //Scaffold has prebuild some widget themes
    return Scaffold(
        backgroundColor: Colors.green[100],
        appBar: AppBar(
          title: Text("My App"),
        ),
        body: data != null
            ? ListView.builder(itemBuilder: (context, index) {
                return ListTile(
                  title: Text(data[index]["title"]),
                );
              })
            : Center(child: CircularProgressIndicator()));
  }
}

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: Homepage(),
    );
  }
}

我不确定这是正确的答案，它只是我的猜测，通过看到上述答案，我想我错过了一件事的主要. dart 页，那就是，

themeData(       
 visualDensity: VisualDensity.adaptivePlatformDensity,
)

快乐飘飘！

在你的标题中，你的数据应该在引号内（""），所以不是：

title: Text(data[index]["title"]),

这样提供：

title: Text("${data[index]["title"]}"),