The method 'salvar' was called on null - flutter

Question

I have a problem with that situation. Can you help me ? I'm taking this error message.
Exception has occurred.
I want save a thing in a list but show:
The following NoSuchMethodError was thrown while handling a gesture:
The method 'salvar' was called on null.
Receiver: null
Tried calling: salvar(Instance of 'Tarefa')
THE CODE:
class TarefaScreen extends StatefulWidget {
#override
_TarefaScreenState createState() => _TarefaScreenState();
}
class _TarefaScreenState extends State<TarefaScreen> {
final _formKey = GlobalKey<FormState>();
TarefaService _tarefaService;
String _titulo;
String _descricao;
DateTime _dataHora;
#override
void initState() {
super.initState();
}
#override
void dispose() {
super.dispose();
}
_save() {
if (_formKey.currentState.validate()) {
_formKey.currentState.save();
Tarefa _tarefa = Tarefa(
titulo: this._titulo,
descricao: this._descricao,
dataHora: this._dataHora);
this._tarefaService.salvar(_tarefa).then((value) {
showInfo("Tarefa adicionada");
Navigator.of(context).pop();
}
);
}
}
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text("Adicionar Tarefa")
,
),
body: Form(
key: _formKey,
child: ListView(
shrinkWrap: true,
children: <Widget>[...
Padding(
padding: const EdgeInsets.only(top: 30, left: 10, right: 10, bottom: 5),
child: RaisedButton(
child: Text("Enviar"),
onPressed: () {
this._save();
},
),
),
],
),
),
);
}
}
And I define addItem here;
class TarefaService {
final TarefaStore tarefaStore;
TarefaService(this.tarefaStore);
Future<List<Tarefa>> buscarTarefas() {
return Future.value(tarefaStore.tarefas);
}
Future<Tarefa> salvar(Tarefa atividade){
tarefaStore.adicionarTarefa(atividade);
return Future.value(atividade);
}
void dispose(){
}
}
Please, help me

Answer 1

You are trying to call the method on the object which is not instantiated so its null in this._tarefaService.salvar(_tarefa). You need to instantiate _tarefaService. You can do it in init()
_tarefaService = new TarefaService(tarefaStore);