Flutter dropdown Error: Either zero or 2 or more [DropdownMenuItem]

dropdown.dart

  Widget dropdownWidget(InputDecoration decoration) {
    var _value = itemList.isEmpty
        ? value
        : itemList.firstWhere((item) => item.value == value.value);

    return DropdownButtonFormField<DropdownItem>(
        decoration: decoration,
        hint: hint != null ? Text(hint) : null,
        items: _itemList,
        value: _value,
        onChanged: onChange);
  }