flutter DropDownButton移除箭头图标？

import 'package:flutter/material.dart';

class CustomDropdownButtonFormField<T> extends FormField<T> {
  CustomDropdownButtonFormField({
    Key key,
    T value,
    List<DropdownMenuItem<T>> items,
    ValueChanged<T> onChanged,
    InputDecoration decoration = const InputDecoration(),
    FormFieldSetter<T> onSaved,
    FormFieldValidator<T> validator,
  }) : super(
          key: key,
          initialValue: value,
          onSaved: onSaved,
          validator: validator,
          builder: (FormFieldState<T> state) {
            return InputDecorator(
              decoration: decoration.copyWith(
                errorText: state.hasError ? state.errorText : null,
              ),
              child: DropdownButtonHideUnderline(
                child: DropdownButton<T>(
                  value: value,
                  items: items,
                  onChanged: onChanged,
                  icon: null,
                ),
              ),
            );
          },
        );
}

// 使用示例
class MyForm extends StatefulWidget {
  @override
  _MyFormState createState() => _MyFormState();
}

class _MyFormState extends State<MyForm> {
  String dropdownValue;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Form(
          child: CustomDropdownButtonFormField<String>(
            value: dropdownValue,
            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'),
              ),
            ],
            onChanged: (value) {
              setState(() {
                dropdownValue = value;
              });
            },
          ),
        ),
      ),
    );
  }
}