Flutter: Error when trying to add data into a Map List

Question

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 lib\main.dart on A37f in debug mode...
Built build\app\outputs\apk\debug\app-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): ════════════════════════════════════════════════════════════════════════════════════════════════════

Answer 1

List<Map<String, String>> productMap;

should be

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

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

Answer 2

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!