title: The method 'salvar' was called on null
body: <p>I have a problem with that situation. Can you help me ? I'm taking this error message.</p>
  
  <p>Exception has occurred.
  I want save a thing in a list but show: </p>
  
  <p>The following NoSuchMethodError was thrown while handling a gesture:
      The method 'salvar' was called on null.
      Receiver: null
      Tried calling: salvar(Instance of 'Tarefa')</p>
  
  <p><strong>THE CODE:</strong></p>
  
  <pre><code>class TarefaScreen extends StatefulWidget {
    @override
    _TarefaScreenState createState() =&gt; _TarefaScreenState();
  }
  
  class _TarefaScreenState extends State&lt;TarefaScreen&gt; {
    final _formKey = GlobalKey&lt;FormState&gt;();
    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: &lt;Widget&gt;[...
  
            Padding(
              padding: const EdgeInsets.only(top: 30, left: 10, right: 10, bottom: 5),
              child: RaisedButton(
                child: Text("Enviar"),
                onPressed: () {
                  this._save();
                },
              ),
            ),
          ],
        ),
      ),
      );
    }
  }
  </code></pre>
  
  <p>And I define addItem here;</p>
  
  <pre><code>class TarefaService {
  
    final TarefaStore tarefaStore;
  
    TarefaService(this.tarefaStore);
  
    Future&lt;List&lt;Tarefa&gt;&gt; buscarTarefas() {
      return Future.value(tarefaStore.tarefas);
    }
  
    Future&lt;Tarefa&gt; salvar(Tarefa atividade){
      tarefaStore.adicionarTarefa(atividade);
      return Future.value(atividade);
    }
  
    void dispose(){
  
    }
  }
  </code></pre>
  
  <p>Please, help me </p>