Flutter FormBuilder下拉验证不起作用

FormBuilder(
  key: _formKey,
  child: Column(
    children: [
      FormBuilderDropdown(
        name: 'dropdown_field',
        decoration: InputDecoration(labelText: '下拉字段'),
        items: ['选项1', '选项2', '选项3']
            .map((option) => DropdownMenuItem(
                  value: option,
                  child: Text('$option'),
                ))
            .toList(),
        onChanged: (value) {
          setState(() {
            // 更新表单状态
            _formKey.currentState.save();
          });
        },
        // 设置验证规则
        validators: [
          FormBuilderValidators.required(),
        ],
      ),
      RaisedButton(
        child: Text('提交'),
        onPressed: () {
          if (_formKey.currentState.validate()) {
            // 表单验证通过
            // 执行提交操作
          }
        },
      ),
    ],
  ),
);