试试这个：

String? currentValue;
var _dropdownItems = <String>['Full-time', "Part-time", "Internship"];
// put the following inside initState() or build()
// start
_dropdownMenuItems = _dropdownItems
        .map((String value) =>
            DropdownMenuItem(child: Text(value), value: value))
        .toList();
// end
DropdownButton(
  hint: Text("Select Job type"),
  value: currentValue,
  isDense: true,
  icon: const Icon(Icons.keyboard_arrow_down),
  items: _dropdownMenuItems,
  onChanged: onChangedCallback,
);
/* ... */
Future<void> onChangedCallback(String? item) async {
  if (item != null) {
    // change this:
    // setState(() => currentValue = item);
    // to: 
    setState(() => currentValue = _dropdownItems.indexOf(item!).toString());
    //
    final url = "my-api-link";
    final body = {"job_type": item};
    final response = await http.post(Uri.parse(url), body: body);
    print(response.body);
  }
}

在下拉菜单中填充项目参数之前，必须设置“值”参数。
参考下面的例子，其中'dropdownValue'被初始化为'list.first'，因为这个我得到了异常。

import 'package:flutter/material.dart';

/// Flutter code sample for [DropdownButton].

const List list = ['One', 'Two', 'Three', 'Four'];

void main() => runApp(const DropdownButtonApp());

class DropdownButtonApp extends StatelessWidget {
  const DropdownButtonApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: const Text('DropdownButton Sample')),
        body: const Center(
          child: DropdownButtonExample(),
        ),
      ),
    );
  }
}

class DropdownButtonExample extends StatefulWidget {
  const DropdownButtonExample({super.key});

  @override
  State createState() => _DropdownButtonExampleState();
}

class _DropdownButtonExampleState extends State {
  String dropdownValue = list.first;

  @override
  Widget build(BuildContext context) {
    return DropdownButton(
      value: dropdownValue,
      icon: const Icon(Icons.arrow_downward),
      elevation: 16,
      style: const TextStyle(color: Colors.deepPurple),
      underline: Container(
        height: 2,
        color: Colors.deepPurpleAccent,
      ),
      onChanged: (String? value) {
        // This is called when the user selects an item.
        setState(() {
          dropdownValue = value!;
        });
      },
      items: list.map>((String value) {
        return DropdownMenuItem(
          value: value,
          child: Text(value),
        );
      }).toList(),
    );
  }
}