Dart Flatter dropdownlist填充速度太慢

我有一个下拉列表，列表项是从webapi获取的。 有时获取json需要一段时间，dropdownlist无法成功绑定。将引发异常：

I/flutter (21159): ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
I/flutter (21159): The following assertion was thrown building ScopedModelDescendant<MainModel>(dirty):
I/flutter (21159): 'package:flutter/src/material/dropdown.dart': Failed assertion: line 481 pos 15: 'value == null ||
I/flutter (21159): items.where((DropdownMenuItem<T> item) => item.value == value).length == 1': is not true.

---

然后正确填充dropdownlist，但当然没有选择任何元素。

可能

itemId

就像一个巨大的字符串，比较它需要很长时间？不，它是小于10000的整数

  Widget _buildDropDownList(MainModel model) {

      isEmpty: itemId == 0,
      child: new DropdownButtonHideUnderline(
        child: new DropdownButton<String>(
          value: itemId != 0 ? itemId .toString() : null,
          isDense: true,
          onChanged: (String newValue) {
            setState(() {
              itemId = model.allItems
                  .where((x) => x.itemId == int.parse(newValue))
                  .first
                  .itemId;
            });
          },
          items: model.allItems.map((Item item) {
            return new DropdownMenuItem<String>(
              value: item.itemId.toString(),
              child: new Text(item.itemName),
            );
          }).toList(),
        ),
      ),
    );
  }

value = null