Mesmo com o controller no TextField quando tiro o foco do campo o valor é removido

class NewTransfer extends StatelessWidget {
  final TextEditingController fieldControllerAccountNumber = TextEditingController();
  final TextEditingController fieldControllerValue = TextEditingController();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Nova Transferência'),
      ),
      body: Container(
        child: Column(
          children: <Widget>[
            Field(
              controller: this.fieldControllerAccountNumber,
              label: 'Número da conta',
              hint: '0000',
            ),
            Field(
              controller: this.fieldControllerValue,
              label: 'Valor',
              hint: '0.00',
              icon: Icons.monetization_on,
            ),
            RaisedButton(
              child: Text('Confirmar'),
              onPressed: () => createTransfer(context),
            ),
          ],
        ),
      ),
    );
  }
}

class Field extends StatelessWidget {
  final TextEditingController controller;
  final String label;
  final String hint;
  final IconData icon;

  Field({
    this.controller,
    this.label,
    this.hint,
    this.icon,
  });

  @override
  Widget build(BuildContext context) {
    return Padding(
      padding: const EdgeInsets.all(16),
      child: TextField(
        controller: this.controller,
        keyboardType: TextInputType.number,
        style: TextStyle(fontSize: 16),
        decoration: InputDecoration(
          icon: this.icon != null ? Icon(this.icon) : null,
          labelText: this.label,
          hintText: this.hint,
        ),
      ),
    );
  }
}