Probably `DropdownSearchBuilder` could have non nullable `selectedItem`

[vasilich6107]

Is your feature request related to a problem? Please describe.

Here is an fn typedef

typedef DropdownSearchBuilder<T> = Widget Function(BuildContext context, T? selectedItem);

Selected item being defined as nullable at the same time input decorator states

isEmpty: getSelectedItem == null,

Describe the solution you'd like This means that the content will not be shown, considering this - selectedItem for DropdownSearchBuilder could be non nullable cause there is no chance it will be shown.

Describe alternatives you've considered

dropdownBuilder: (_, item) {
        if (item == null) {
          return const SizedBox.shrink();
        }
        return Row(
          children: [
            ...item
          ],
        );
      },

[salim-lachdhaf]

DropdownSearchBuilder should be a nullable selectedItem otherwise, it will be impossible to represent "nothing" value .

isEmpty only affects the display of the hint or label in the dropdown.

Behavior:

• By default: The hint appears inside the text field if the selected item is null. If an item is selected, the hint is displayed as a label.
• When using dropdownBuilder: The behavior depends on how the developer handles null values.

Examples:

1. If the developer handles null as follows, the hint remains inside the text field without issue:

   dropdownBuilder: (_, item) {
    if (item == null) {
      return const SizedBox.shrink();
    }
    return Row(
      children: [
        ...item
      ],
    );
   },

2. If the developer uses a non-empty or visible widget for a null item, like this:

   dropdownBuilder: (_, item) {
    if (item == null) {
      return Text('No item selected');
    }
    return Row(
      children: [
        ...item
      ],
    );
   },

   In this case, the hint is hidden by default, and the developer should use floatingLabelBehavior to explicitly show the hint as a label.

For more context, you can refer to this related topic: GitHub issue #515.