How to make condition on dropdownMenu

I need advice. I have made condition on dropdownMenu, at first it works, but since I made a change in my code, it didn’t work again.
The problem is the user cannot choose the menu.

You can look at the UI that I attach.

Sorry I can’t make a record with video.

And here is my code that I made so far:

There is a variable below BuildContext, far in the top:

Widget build(BuildContext context) {
String selectedCategoryArea = '';

Then this is the dropdownMenu:

child: DropdownButton<String>(
                        icon: Padding(
                          padding: const EdgeInsets.only(right: 10, top: 8),
                          child: SvgPicture.asset(
                            Assets.icons.dropdownIcon.path,
                            fit: BoxFit.scaleDown,
                          ),
                        ),
                        style: body1(color: ColorName.grey),
                        items: <String>[
                          'Block',
                          'Fining Line',
                        ].map((String value) {
                          return DropdownMenuItem(
                            value: value,
                            child: Text(value),
                          );
                        }).toList(),
                        hint: Padding(
                          padding: const EdgeInsets.only(top: 8, left: 10),
                          child: Text(
                              style: body1(color: ColorName.grey),
                              selectedCategoryArea.isEmpty
                                  ? 'Category Area'
                                  : selectedCategoryArea),
                        ),
                        borderRadius: BorderRadius.circular(10),
                        underline: const SizedBox(),
                        isExpanded: true,
                        onChanged: (value) {
                          if (value != null) {
                            setState(() {
                              selectedCategoryArea = value;
                            });
                          }
                        },
                      ),

>Solution :

The problem is that you have defined selectedCategoryArea within your build method:

Widget build(BuildContext context) {
var selectedCategoryArea = "";

So, every setState it gets reset.

To fix the issue, declare selectedCategoryArea in your _state class:

class _TestState extends State<Test> {
  var selectedCategoryArea = "";
  @override
  Widget build(BuildContext context) {
    return Scaffold(