Ggtcf

I am very much new to Flutter. I am developing a testing app and blow is my code.

main.dart

import 'package:flutter/material.dart';

import './pages/homepage.dart';

void main() 
 runApp(MyApp());


class MyApp extends StatelessWidget 
 @override
 Widget build(BuildContext context) 
 // TODO: implement build
 return MaterialApp(
 theme: ThemeData(primaryColor: Colors.green),
 home: Homepage(),
 );
 

homepage.dart

import 'package:flutter/material.dart';

class Homepage extends StatelessWidget 
 List<Map<String, String>> productMap;

 Homepage() 
 productMap.add("title": "Chocolate", "imageUrl": "");
 

 @override
 Widget build(BuildContext context) 
 // TODO: implement build
 return Scaffold(
 appBar: AppBar(
 title: Text("Choco Factory"),
 ),
 body: HomepageUI(),
 );
 


class HomepageUI extends StatefulWidget 
 HomepageUI() 

 @override
 State<StatefulWidget> createState() 
 // TODO: implement createState
 return _HomepageUIBuilder();
 


class _HomepageUIBuilder extends State<HomepageUI> 
 @override
 Widget build(BuildContext context) 
 // TODO: implement build
 return Expanded(
 child: Column(
 children: <Widget>[
 ListView.builder(
 itemBuilder: _listBuilder,
 )
 ],
 ),
 );
 

 Widget _listBuilder(BuildContext context, int index) 
 return Card(
 child: Column(
 children: <Widget>[
 Image.asset("name"),
 Text("data"),
 RaisedButton(
 onPressed: () ,
 child: Text("Details"),
 color: ThemeData().primaryColor,
 )
 ],
 ),
 );
 

I get the following error and I can't figure out why. I am from Java background and the code seems OK to me. Also I have another question, in Java Map we can add many key-value pairs to the Map but in Flutter it seems only one key-value pair is possible?

Launching libmain.dart on A37f in debug mode...
Built buildappoutputsapkdebugapp-debug.apk.
I/Choreographer(11935): Skipped 87 frames! The application may be doing too much work on its main thread.
I/flutter (11935): ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
I/flutter (11935): The following NoSuchMethodError was thrown building MyApp(dirty):
I/flutter (11935): The method 'add' was called on null.
I/flutter (11935): Receiver: null
I/flutter (11935): Tried calling: add(_LinkedHashMap len:2)
I/flutter (11935): When the exception was thrown, this was the stack:
I/flutter (11935): #0 Object.noSuchMethod (dart:core/runtime/libobject_patch.dart:50:5)
I/flutter (11935): #1 new Homepage 
I/flutter (11935): #2 MyApp.build 
I/flutter (11935): #3 StatelessElement.build 
I/flutter (11935): #4 ComponentElement.performRebuild 
I/flutter (11935): #5 Element.rebuild 
I/flutter (11935): #6 ComponentElement._firstBuild 
I/flutter (11935): #7 ComponentElement.mount 
I/flutter (11935): #8 Element.inflateWidget 
I/flutter (11935): #9 Element.updateChild 
I/flutter (11935): #10 RenderObjectToWidgetElement._rebuild 
I/flutter (11935): #11 RenderObjectToWidgetElement.mount 
I/flutter (11935): #12 RenderObjectToWidgetAdapter.attachToRenderTree.<anonymous closure> 
I/flutter (11935): #13 BuildOwner.buildScope 
I/flutter (11935): #14 RenderObjectToWidgetAdapter.attachToRenderTree 
I/flutter (11935): #15 _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding&PaintingBinding&SemanticsBinding&RendererBinding&WidgetsBinding.attachRootWidget 
I/flutter (11935): #16 runApp 
I/flutter (11935): #17 main 
I/flutter (11935): #18 _runMainZoned.<anonymous closure>.<anonymous closure> (dart:ui/hooks.dart:189:25)
I/flutter (11935): #23 _runMainZoned.<anonymous closure> (dart:ui/hooks.dart:180:5)
I/flutter (11935): #24 _startIsolate.<anonymous closure> (dart:isolate/runtime/libisolate_patch.dart:300:19)
I/flutter (11935): #25 _RawReceivePortImpl._handleMessage (dart:isolate/runtime/libisolate_patch.dart:171:12)
I/flutter (11935): (elided 4 frames from package dart:async)
I/flutter (11935): ════════════════════════════════════════════════════════════════════════════════════════════════════

List<Map<String, String>> productMap;

should be

List<Map<String, String>> productMap = [];

otherwise productMap is only declared, but not initialized (null)

You have not initialised the List.

Please change

List<Map<String, String>> productMap;

to

List<Map<String, String>> productMap = <Map<String, String>>[];

Hope this helped!