Flutter DropdownButton With Validation

flutter-dropdown.dart

 String _town; 
  _townField() {
    return FormField<String>(
      validator: (value) {
        if (value == null) {
          return "Select your area";
        }
      },
      onSaved: (value) {
        formData['town'] = value;
      },
      builder: (
        FormFieldState<String> state,
      ) {
        return Column(
          crossAxisAlignment: CrossAxisAlignment.start,
          children: <Widget>[
            new InputDecorator(
              decoration: const InputDecoration(
                contentPadding: EdgeInsets.all(0.0),
                labelText: 'Area',
              ),
              child: DropdownButtonHideUnderline(
                child: DropdownButton<String>(
                  hint: new Text("Select Town"),
                  value: _town,
                  onChanged: (String newValue) {
                    state.didChange(newValue);
                    setState(() {
                      _town = newValue;
                    });
                  },
                  items: <String>[
                    'Sukhchayn Garden',
                    'Canal Garden',
                    'Bahria Town',
                  ].map((String value) {
                    return new DropdownMenuItem<String>(
                      value: value,
                      child: new Text(value),
                    );
                  }).toList(),
                ),
              ),
            ),
            SizedBox(height: 5.0),
            Text(
              state.hasError ? state.errorText : '',
              style:
                  TextStyle(color: Colors.redAccent.shade700, fontSize: 12.0),
            ),
          ],
        );
      },
    );
  }