我最近使用的下拉菜单如下
第一步

List<String> listOfPromote = [
     'My biggest work achievement is',
     'An epic work fail i had was',
];

第2步

String? promoteValue;

第3步（创建下拉按钮表单字段）

DropdownButtonFormField(
      decoration: InputDecoration(
        enabledBorder: UnderlineInputBorder(
          borderSide: BorderSide(
              color: Color(0xFFF44336)),
        ),
        focusedBorder: const UnderlineInputBorder(
            borderSide: BorderSide(color: Colors.grey),
        errorBorder: UnderlineInputBorder(
            borderSide: BorderSide(
                color: Color(0xFFF44336))),
      ),
      isDense: false,
      value: experienceValue,
      hint: Text(
        'Select an experience',
        style: Textstyle.copyWith(
          color: Color.withAlpha(0xFFF44336),
        ),
      ),
      onChanged: (value) {
        setState(() {
          experienceValue = value;
          //print(experienceValue);
        });
      },
      onSaved: (value) {
        setState(() {
          experienceValue = value;
        });
      },
      validator: (String? value) {
        if (value!.isEmpty) {
          return "can't empty";
        } else {
          return null;
        }
      },
      items: listOfExperience.map((String val) {
        return DropdownMenuItem(
          value: val,
          child: Text(val),
        );
      }).toList(),
    ),
  ),

我希望这将有助于理解和执行您的一个。如果它不是，然后评论。