您需要放置空值而不是空字符串

setstate({
  dropdownValue = null ;
});

如果你不喜欢null，就把它放在那里，并且确保在build方法之外有StatefulWidget和变量。
完整小部件

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

  @override
  State<drawarScreen> createState() => _drawarScreenState();
}

class _drawarScreenState extends State<drawarScreen> {
  List<String> list = <String>['One', 'Two', 'Three', 'Four'];
  String? dropdownValue;
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Column(
        children: [
          DropdownButton<String>(
            hint: Text('moammed'),
            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) {
              setState(() {
                dropdownValue = value;
              });
            },
            items: list.map<DropdownMenuItem<String>>((String value) {
              return DropdownMenuItem<String>(
                value: value,
                child: Text(value),
              );
            }).toList(),
          ),
          ElevatedButton(
              onPressed: () {
                setState(() {
                  dropdownValue = null;
                });
              },
              child: Text("Reset"))
        ],
      ),
    );
  }
}