Fix DropdownButton bug (#65915)

packages/flutter/lib/src/material/dropdown.dart

@@ -1303,16 +1303,12 @@ class _DropdownButtonState<T> extends State<DropdownButton<T>> with WidgetsBindi
 
     // The width of the button and the menu are defined by the widest
     // item and the width of the hint.
-    List<Widget> items;
-    if (_enabled) {
-      items = widget.selectedItemBuilder == null
-        ? List<Widget>.from(widget.items!)
-        : widget.selectedItemBuilder!(context);
-    } else {
-      items = widget.selectedItemBuilder == null
-        ? <Widget>[]
-        : widget.selectedItemBuilder!(context);
-    }
+    // We should explicitly type the items list to be a list of <Widget>,
+    // otherwise, no explicit type adding items maybe trigger a crash/failure
+    // when hint and selectedItemBuilder are provided.
+    final List<Widget> items = widget.selectedItemBuilder == null
+      ? (_enabled ? List<Widget>.from(widget.items!) : <Widget>[])
+      : List<Widget>.from(widget.selectedItemBuilder!(context));
 
     int? hintIndex;
     if (widget.hint != null || (!_enabled && widget.disabledHint != null)) {

packages/flutter/test/material/dropdown_test.dart

@@ -1945,7 +1945,7 @@ void main() {
                   });
                 },
                 selectedItemBuilder: (BuildContext context) {
-                  return items.map<Widget>((String item) {
+                  return items.map((String item) {
                     return Text('You have selected: $item');
                   }).toList();
                 },