Flutter - The method was called on null

Question

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     gt  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     gt  new  MyHomePageState       class  MyHomePageState extends State lt MyHomePage gt    CryptoListPresenter  presenter    List lt Crypto gt   currencies    bool  isLoading    final List lt MaterialColor gt   colors    Colors blue  Colors indigo  Colors red   List lt String gt  items      Item 1    Item 2    Item 3    Item 4    Item 5    Item 6    Item 7    Item 8          var refreshKey   new GlobalKey lt RefreshIndicatorState gt         override   void initState         super initState         isLoading   true       presenter loadCurrencies        refreshList             Future lt Null gt  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   gt  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   lt Widget gt                 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   lt Widget gt                           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   lt Widget gt         new IconButton icon  new Icon Icons shopping cart  color  Colors white    onPressed                      backgroundColor  Colors transparent    body  new Container      child  new Stack        children   lt Widget gt           new Padding            padding  new EdgeInsets only top  10 0             child  new Column              crossAxisAlignment  CrossAxisAlignment center              mainAxisSize  MainAxisSize max              mainAxisAlignment  MainAxisAlignment start              children   lt Widget gt                 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   lt Widget gt                         new Row                          crossAxisAlignment  CrossAxisAlignment start                          children   lt Widget gt                                  new Container                                  margin  const EdgeInsets all 10 0                                     child  new Column                                      children   lt Widget gt                                         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   lt Widget gt                                     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   lt Widget gt           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

User · Answer

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

User · Answer

You have a CryptoListPresenter  presenter but you are never initializing it  You should either be doing that when you declare it or in your initState    or another appropriate but called-before-you-need-it method    One thing I find that helps is that if I know a member is functionally  final   to actually set it to final as that way the analyzer complains that it hasn t been initialized   EDIT    I see diegoveloper beat me to answering this  and that the OP asked a follow up    Jake - it s hard for us to tell without knowing exactly what CryptoListPresenter is  but depending on what exactly CryptoListPresenter actually is  generally you d do final CryptoListPresenter  presenter   new CryptoListPresenter        or   CryptoListPresenter  presenter    override void initState        presenter   new CryptoListPresenter

User · Answer

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

User · Answer

Because of your initialization wrong    Don t do like this   MethodName  methodName   Do like this   MethodName  methodName   MethodName

User · Answer

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  Can be initialized during an declaration  like this CryptoListPresenter  presenter   CryptoListPresenter      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

[flutter] Flutter - The method was called on null

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