In flutter, json data is not appearing in listview initially - flutter

Question

At first, the list of publications was shown in ListTile. However, when I changed the String query, it doesn't show any data at all, and I can't search it. I want to show it as a ListView, but I want to hide it before searching and enter the correct title when I search, and I want to see what I searched. It's strange why suddenly a JSON file comes out.
import 'package:flutter/material.dart';
import 'dart:async';
import 'package:http/http.dart' as http;
import 'dart:convert';
import 'package:shared_preferences/shared_preferences.dart';
void main() {
runApp(MaterialApp(
home: MyGetHttpData(),
));
}
class MyGetHttpData extends StatefulWidget {
#override
MyGetHttpDataState createState() => MyGetHttpDataState();
}
class MyGetHttpDataState extends State<MyGetHttpData> {
String query = '유시민';
int page = 1;
String url;
List data;
Future<String> getJSONData() async {
var response = await http.get(
Uri.encodeFull(url),
headers: {"Accept": "application/json" , 'Authorization' : 'KakaoAK 5a1167ba90ccb4b79d84ef5a41da0fbd'});
//REST API 여기에 입력//
print(response.body);
setState(() {
var dataConvertedToJSON = json.decode(response.body);
data = dataConvertedToJSON['documents'];
});
return "Successfull";
}
TextEditingController editingController = TextEditingController();
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text('widgettitle'),
),
body: Container(
child: Column(
children: <Widget>[
Padding(
padding: const EdgeInsets.all(8.0),
child: TextField(
onChanged: (value) {
},
controller: editingController,
decoration: InputDecoration(
labelText: "Search",
hintText: "Search",
prefixIcon: Icon(Icons.search),
border: OutlineInputBorder(
borderRadius: BorderRadius.all(Radius.circular(25.0)))),
),
),
Expanded(
child: ListView.builder(
shrinkWrap: true,
itemCount: data == null ? 0 : data.length,
itemBuilder: (context, index) {
return ListTile(
title: Text(
data[index]['publisher'].toString(),
style: TextStyle(
fontSize: 20.0, color: Colors.black, ),
),
);
},
),
),
],
),
),
);
}
}
SharedPreferences prefs;
List<String> list;
// ...
Future<bool> _saveList() async {
return await prefs.setStringList("key", list);
}
List<String> _getList() {
return prefs.getStringList("key");
}
#override
void initState() {
String query = '유시민';
int page = 1;
String url;
List data;
url = 'https://dapi.kakao.com/v3/search/book?sort=accuracy&target=title&size=10&query= $query &page= $page';
}