The Method [] was called on null

I am new to Flutter. I seen many tutorials on YouTube and Google but I can't find solution on this. How can I resolve this error?

import 'package:flutter/material.dart'; import 'dart:ui'; import 'dart:convert'; import 'data/shelfscreendata.dart'; import 'package:google_fonts/google_fonts.dart'; import 'constant/constant.dart' as Constants; import 'PhotographerWelcomeNote.dart'; import 'package:http/http.dart' as http; import 'Utility/global.dart' as Globals; import 'data/errordata.dart'; import 'data/shelfscreendata.dart'; import 'data/shelfscreendata.dart'; class ShelfScreen extends StatefulWidget { @override _ShelfScreenState createState() => _ShelfScreenState(); } class _ShelfScreenState extends State<ShelfScreen> { bool _isLoading = false; Map CompaignShelfList; List CampiagnData; Future Shelf() async { Map data = { "AccessToken": "MjUyLTg1REEyUzMtQURTUzVELUVJNUI0QTIyMTE=", "CustomerId": 1 }; final http.Response response = await http.post( "http://api.pgapp.in/v1/shelflist", headers: <String, String>{ 'Content-Type': 'application/json; charset=UTF-8', }, body: jsonEncode(data), ); var jsonResponse = null; CompaignShelfList = json.decode(response.body); setState(() { CampiagnData = CompaignShelfList["CompaignShelfList"]; }); // debugPrint(CompaignShelfList.toString()); if (response.statusCode == 200) { jsonResponse = json.decode(response.body); print("Response status : ${response.statusCode}"); print("Response status : ${response.body}"); if(jsonResponse != null && ! jsonResponse.containsKey("Error")){ setState(() { _isLoading = false; }); } else{ setState(() { _isLoading = false; }); print("Response status : ${response.body}"); } } } @override void initState() { super.initState(); Shelf(); } @override Widget build(BuildContext context) { return Scaffold( appBar: AppBar( backgroundColor: const Color(0xfff8f8f8), leading: Padding( padding: EdgeInsets.only(left: 20), child: Image.asset("assets/logo.png",width: 45,height: 45,), ), title: Center( child: Image.asset("assets/Textimage.png",width: 170,height: 45), ), actions: <Widget>[ Padding( padding: const EdgeInsets.only(right: 20,top: 10,bottom: 10), child: Container( height: 8, width: 35, child: CircleAvatar( maxRadius: 20, backgroundImage: AssetImage("assets/images/prof.jpg"), ), ), ), ], ), body: GridView.builder( shrinkWrap: false, scrollDirection: Axis.vertical, gridDelegate: new SliverGridDelegateWithFixedCrossAxisCount( crossAxisCount: 1, childAspectRatio: 1.200, mainAxisSpacing: 1.0, crossAxisSpacing: 1.0, ), itemCount: CompaignShelfList.length, itemBuilder: (context, int index){ return CategoriesTile( imgUrls: CompaignShelfList[index]["CoverImagePath"], catagory:CompaignShelfList[index]["CampaignTitle"], date: CompaignShelfList[index]["Date"], email: CompaignShelfList[index]["PhotographerEmail"], stduio:CompaignShelfList[index]["BusinessName"], ); // Image.asset(Shelflist[index]["text_3"],fit: BoxFit.cover,); } ) ); } } class CategoriesTile extends StatelessWidget { final String imgUrls, catagory , date,email,stduio; CategoriesTile({@required this.imgUrls, @required this.catagory,@required this.date,@required this.email,@required this.stduio}); @override Widget build(BuildContext context) { return GestureDetector( onTap: () { Navigator.push( context, MaterialPageRoute( builder: (context) => PhotographerWelcomeNote( Coverpic: imgUrls, ))); }, child: Card( semanticContainer: true, clipBehavior: Clip.antiAliasWithSaveLayer, elevation: 5, child: Container( margin: EdgeInsets.all(8), child: Stack( children: <Widget>[ Positioned( top: 1, left: 1, right: 1, bottom: 50, child: ClipRRect( borderRadius: BorderRadius.circular(8), child:Image.asset( imgUrls, height: 250, width: 400, fit: BoxFit.cover, ) ), ), Positioned( bottom: 50, left: 1, right: 1, // width: 400, height: 60, child: ClipRect( child: BackdropFilter( filter: ImageFilter.blur(sigmaX: 3, sigmaY: 3), child: Container( decoration: BoxDecoration( color: Colors.black.withOpacity(0), borderRadius: BorderRadius.circular(8), ), ), ), ), ), Positioned( bottom: 80, left: 20, child: Container( height: 50, width: 400, alignment: Alignment.bottomLeft, child: Text( catagory ?? "Yo Yo", style: GoogleFonts.nunito( fontSize: 18, color: const Color(0xffffffff), fontWeight: FontWeight.w700, height: 1.7647058823529411, ), )), ), Positioned( left: 20, bottom: 60, child: Container( height: 30, width: 150, alignment: Alignment.bottomLeft, child: Text( date,style: GoogleFonts.nunito( fontSize: 13, color: const Color(0xffffffff), fontWeight: FontWeight.w700, height: 1.7647058823529411, ), ), ), ), Positioned( bottom: 20, left: 90, child: Container( height: 23, alignment: Alignment.center, child: Text( stduio, style: GoogleFonts.nunito( fontSize: 18, color: const Color(0xff7f7f7f), fontWeight: FontWeight.w700, ) ), ), ), Positioned( bottom: 1, left: 90, child: Container( height: 20, alignment: Alignment.center, child: Text( email,style: GoogleFonts.nunito( fontSize: 10, color: const Color(0xffb4b4b4), fontWeight: FontWeight.w600, ), ), ), ), Positioned( bottom: 2, left: 10, child: Container( width: 66.0, height: 39.0, decoration: BoxDecoration( image: DecorationImage( image: const AssetImage( 'assets/images/camlogo.jpg'), fit: BoxFit.cover, ), border: Border.all( width: 1.0, color: const Color(0xffb4b4b4)), ), ), ), ], ), ), ), ); } }