无法使用getx obs更新flutter中的DropdownButtonFormField字段值

dependencies:
  get: ^4.1.4

import 'package:get/get.dart';

class DropdownController extends GetxController {
  var dropdownValue = 'Option 1'.obs;
}

class MyPage extends StatelessWidget {
  final DropdownController dropdownController = Get.put(DropdownController());

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('My Page'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            Obx(() => DropdownButtonFormField<String>(
                  value: dropdownController.dropdownValue.value,
                  onChanged: (newValue) {
                    dropdownController.dropdownValue.value = newValue;
                  },
                  items: [
                    DropdownMenuItem(
                      value: 'Option 1',
                      child: Text('Option 1'),
                    ),
                    DropdownMenuItem(
                      value: 'Option 2',
                      child: Text('Option 2'),
                    ),
                    DropdownMenuItem(
                      value: 'Option 3',
                      child: Text('Option 3'),
                    ),
                  ],
                )),
          ],
        ),
      ),
    );
  }
}