fix a NestedScrollView's ScrollPosition access bug (#80573)

packages/flutter/lib/src/widgets/nested_scroll_view.dart

@@ -1317,6 +1317,7 @@ class _NestedScrollController extends ScrollController {
   @override
   void detach(ScrollPosition position) {
     assert(position is _NestedScrollPosition);
+    (position as _NestedScrollPosition).setParent(null);
     position.removeListener(_scheduleUpdateShadow);
     super.detach(position);
     _scheduleUpdateShadow();
@@ -1621,12 +1622,6 @@ class _NestedScrollPosition extends ScrollPosition implements ScrollActivityDele
   Drag drag(DragStartDetails details, VoidCallback dragCancelCallback) {
     return coordinator.drag(details, dragCancelCallback);
   }
-
-  @override
-  void dispose() {
-    _parent?.detach(this);
-    super.dispose();
-  }
 }
 
 enum _NestedBallisticScrollActivityMode { outer, inner, independent }

packages/flutter/test/widgets/nested_scroll_view_test.dart

@@ -2342,6 +2342,53 @@ void main() {
     expect(isScrolled, false);
     expect(tester.takeException(), isNull);
   });
+
+  // Regression test of https://github.com/flutter/flutter/issues/74372
+  testWidgets('ScrollPosition can be accessed during `_updatePosition()`', (WidgetTester tester) async {
+    final ScrollController controller = ScrollController();
+    late ScrollPosition position;
+
+    Widget buildFrame({ScrollPhysics? physics}) {
+      return Directionality(
+        textDirection: TextDirection.ltr,
+        child: Localizations(
+          locale: const Locale('en', 'US'),
+          delegates: const <LocalizationsDelegate<dynamic>>[
+            DefaultMaterialLocalizations.delegate,
+            DefaultWidgetsLocalizations.delegate,
+          ],
+          child: MediaQuery(
+            data: const MediaQueryData(),
+            child: NestedScrollView(
+              controller: controller,
+              physics: physics,
+              headerSliverBuilder: (BuildContext context, bool innerBoxIsScrolled) {
+                return <Widget>[
+                  Builder(
+                    builder: (BuildContext context) {
+                      position = controller.position;
+                      return const SliverAppBar(
+                        floating: true,
+                        title: Text('AA'),
+                      );
+                    },
+                  ),
+                ];
+              },
+              body: Container(),
+            ),
+          ),
+        ),
+      );
+    }
+
+    await tester.pumpWidget(buildFrame());
+    expect(position.pixels, 0.0);
+
+    //Trigger `_updatePosition()`.
+    await tester.pumpWidget(buildFrame(physics: const _CustomPhysics()));
+    expect(position.pixels, 0.0);
+  });
 }
 
 class TestHeader extends SliverPersistentHeaderDelegate {