[flutter] Flutter - The method was called on null

I'm programming an app that displays two media streams in the same view via one horizontal listView and one vertical listView. I'm currently working on implementing information into the bottom listView using some nice looking boilerplate that can be found here.

I'm new to flutter and have believe my code has gotten a little messy, in short I'm receiving the error;

flutter: The following NoSuchMethodError was thrown building Builder:
flutter: The method 'loadCurrencies' was called on null.
flutter: Receiver: null
flutter: Tried calling: loadCurrencies()

when trying to implement code found here in the home_page_view.dart into the aforementioned boilerplate.

This is the code I'm using to stimulate the error;

import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';
import 'package:to_be_deleted/data/crypto_data.dart';
import 'package:to_be_deleted/modules/crypto_presenter.dart';
import 'dart:async';
import 'dart:io';
import 'background.dart';

void main() => runApp(new MyApp());

class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return new MaterialApp(
  title: 'Flutter Demo',
  theme: new ThemeData(
    primarySwatch: Colors.blue,
  ),
  debugShowCheckedModeBanner: false,
  home: new MyHomePage(title: 'Popular'),
);
}
}

class MyHomePage extends StatefulWidget {
MyHomePage({Key key, this.title}) : super(key: key);

final String title;

@override
_MyHomePageState createState() => new _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
CryptoListPresenter _presenter;
  List<Crypto> _currencies;
  bool _isLoading;
  final List<MaterialColor> _colors = [Colors.blue, Colors.indigo, Colors.red];
List<String> items = [
"Item 1",
"Item 2",
"Item 3",
"Item 4",
"Item 5",
"Item 6",
"Item 7",
"Item 8"
];



  var refreshKey = new GlobalKey<RefreshIndicatorState>();

  @override
  void initState() {
    super.initState();
    _isLoading = true;
    _presenter.loadCurrencies();
    refreshList();
  }

    Future<Null> refreshList() async {
    refreshKey.currentState?.show(atTop: false);
    await new Future.delayed(new Duration(seconds: 1));
      _presenter.loadCurrencies();
      Scaffold.of(context).showSnackBar(
            new SnackBar(
              duration: new Duration(seconds: 3),
              backgroundColor: Colors.black,
              content: new Text(
                    'Refresh available every 60 seconds',
                    textAlign: TextAlign.center,
                  ),
              ),
            );
    setState(() {
    });
    return null;
  }

@override
Widget build(BuildContext context) {
final _width = MediaQuery.of(context).size.width;
final _height = MediaQuery.of(context).size.height;

  Widget _getSubtitleText(String priceUSD, String percentageChange) {
    TextSpan priceTextWidget = new TextSpan(
        text: "\$$priceUSD\n", style: new TextStyle(color: Colors.black));
    String percentageChangeText = "1 hour: $percentageChange%";
    TextSpan percentageChangeTextWidget;

    if (double.parse(percentageChange) > 0) {
      percentageChangeTextWidget = new TextSpan(
          text: percentageChangeText,
          style: new TextStyle(color: Colors.green ));
    } else {
      percentageChangeTextWidget = new TextSpan(
          text: percentageChangeText, style: new TextStyle(color: Colors.red));
    }

    return new RichText(
        text: new TextSpan(
            children: [priceTextWidget, percentageChangeTextWidget]));
  }

  ListTile _getListItemUi(Crypto currency, MaterialColor color) {
    return new ListTile(
      leading: new Image.asset(
        "cryptoiconsBlack/"+currency.symbol.toLowerCase()+"@2x.png",
        width: 64.0,
        height: 64.0,
        ),
      title: new Text(currency.name,
          style: new TextStyle(fontWeight: FontWeight.bold)),
      subtitle:
      _getSubtitleText(currency.price_usd, currency.percent_change_1h),
      isThreeLine: true,
    );
  }

final headerList = new ListView.builder(
  itemBuilder: (context, index) {
    EdgeInsets padding = index == 0?const EdgeInsets.only(
        left: 20.0, right: 10.0, top: 4.0, bottom: 30.0):const EdgeInsets.only(
        left: 10.0, right: 10.0, top: 4.0, bottom: 30.0);

    return new Padding(
      padding: padding,
      child: new InkWell(
        onTap: () {
          print('Card selected');
        },
        child: new Container(
          decoration: new BoxDecoration(
            borderRadius: new BorderRadius.circular(10.0),
            color: Colors.lightGreen,
            boxShadow: [
              new BoxShadow(
                  color: Colors.black.withAlpha(70),
                  offset: const Offset(3.0, 10.0),
                  blurRadius: 15.0)
            ],
            image: new DecorationImage(
              image: new ExactAssetImage(
                  'assets/img_${index%items.length}.jpg'),
              fit: BoxFit.fitHeight,
            ),
          ),
        //height: 200.0,
          width: 200.0,
          child: new Stack(
            children: <Widget>[
              new Align(
                alignment: Alignment.bottomCenter,
                child: new Container(
                    decoration: new BoxDecoration(
                        color: const Color(0xFF273A48),
                        borderRadius: new BorderRadius.only(
                            bottomLeft: new Radius.circular(10.0),
                            bottomRight: new Radius.circular(10.0))),
                    height: 30.0,
                    child: new Row(
                      mainAxisAlignment: MainAxisAlignment.center,
                      children: <Widget>[
                        new Text(
                          '${items[index%items.length]}',
                          style: new TextStyle(color: Colors.white),
                        )
                      ],
                    )),
              )
            ],
          ),
        ),
      ),
    );
  },
  scrollDirection: Axis.horizontal,
  itemCount: items.length,
);

final body = new Scaffold(
  appBar: new AppBar(
    title: new Text(widget.title),
    elevation: 0.0,
    backgroundColor: Colors.transparent,
    actions: <Widget>[
      new IconButton(icon: new Icon(Icons.shopping_cart, color: Colors.white,), onPressed: (){})
    ],
  ),
  backgroundColor: Colors.transparent,
  body: new Container(
    child: new Stack(
      children: <Widget>[
        new Padding(
          padding: new EdgeInsets.only(top: 10.0),
          child: new Column(
            crossAxisAlignment: CrossAxisAlignment.center,
            mainAxisSize: MainAxisSize.max,
            mainAxisAlignment: MainAxisAlignment.start,
            children: <Widget>[
              new Align(
                alignment: Alignment.centerLeft,
                child: new Padding(
                    padding: new EdgeInsets.only(left: 8.0),
                    child: new Text(
                      'Trending News',
                      style: new TextStyle(color: Colors.white70),
                    )),
              ),
              new Container(
                  height: 300.0, width: _width, child: headerList),
              new Expanded(child:
              ListView.builder(itemBuilder: (context, index) {
                return new ListTile(
                  title: new Column(
                    children: <Widget>[
                      new Row(
                        crossAxisAlignment: CrossAxisAlignment.start,
                        children: <Widget>[
                               new Container(
                                margin: const EdgeInsets.all(10.0),
                                  child: new Column(
                                    children: <Widget>[
                                      new Flexible(
                                        child: new ListView.builder(
                                          itemCount: _currencies.length,
                                          itemBuilder: (BuildContext context, int index) {
                                            final int i = index ~/ 2;
                                            final Crypto currency = _currencies[i];
                                            final MaterialColor color = _colors[i % _colors.length];
                                            if (index.isOdd) {
                                              return new Divider(color: Colors.grey);
                                            }
                                            return _getListItemUi(currency, color);
                                          },
                                        ),
                                      )
                                    ],
                                  )
                                ),
                          new SizedBox(
                            width: 8.0,
                          ),
                          new Expanded(
                              child: new Column(
                                mainAxisAlignment:
                                MainAxisAlignment.start,
                                crossAxisAlignment:
                                CrossAxisAlignment.start,
                                children: <Widget>[
                                  new Text(
                                    'My item header',
                                    style: new TextStyle(
                                        fontSize: 14.0,
                                        color: Colors.black87,
                                        fontWeight: FontWeight.bold),
                                  ),
                                  new Text(
                                    'Item Subheader goes here\nLorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry',
                                    style: new TextStyle(
                                        fontSize: 12.0,
                                        color: Colors.black54,
                                        fontWeight: FontWeight.normal),
                                  )
                                ],
                              )),
                          new Icon(
                            Icons.shopping_cart,
                            color: const Color(0xFF273A48),
                          )
                        ],
                      ),
                      new Divider(),
                    ],
                  ),
                );
              }))
            ],
          ),
        ),
      ],
    ),
  ),
);








  return new Container(
    decoration: new BoxDecoration(
      color: const Color(0xFF273A48),
    ),
    child: new Stack(
      children: <Widget>[
        new CustomPaint(
          size: new Size(_width, _height),
          painter: new Background(),
        ),
        body,
      ],
    ),
  );
  }
}

This is what my simulator looks like after I've Initialised the app

Thanks for the help, and if you need any more code please let me know.

The reason for this error occurs is that you are using the CryptoListPresenter _presenter without initializing.

I found that CryptoListPresenter _presenter would have to be initialized to fix because _presenter.loadCurrencies() is passing through a null variable at the time of instantiation;

there are two ways to initialize

1. Can be initialized during an declaration, like this

   CryptoListPresenter _presenter = CryptoListPresenter();
   

2. In the second, initializing(with assigning some value) it when initState is called, which the framework will call this method once for each state object.

   @override
   void initState() {
     _presenter = CryptoListPresenter(...);
   }
   

As stated in the above answers, it's always a good practice to initialize the variables, but if you have something which you don't know what value should it takes, and you want to leave it uninitialized so you have to make sure that you are updating it before using it.

For example: Assume we have double _bmi; and you don't know what value should it takes, so you can leave it as it is, but before using it, you have to update its value first like calling a function that calculating BMI like follows:

String calculateBMI (){
_bmi = weight / pow( height/100, 2);
return _bmi.toStringAsFixed(1);}

or whatever, what I mean is, you can leave the variable as it is, but before using it make sure you have initialized it using whatever the method you are using.

Because of your initialization wrong.

Don't do like this,

MethodName _methodName;

Do like this,

MethodName _methodName = MethodName();

You should declare your method first in void initState(), so when the first time pages has been loaded, it will init your method first, hope it can help