[Scrollbar]Skip the ScrollPosition check if the bar was unmounted (#103948)

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

@@ -1447,6 +1447,9 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
   }
 
   bool _debugCheckHasValidScrollPosition() {
+    if (!mounted) {
+      return true;
+    }
     final ScrollController? scrollController = widget.controller ?? PrimaryScrollController.of(context);
     final bool tryPrimary = widget.controller == null;
     final String controllerForError = tryPrimary

packages/flutter/test/widgets/scrollbar_test.dart

@@ -2444,4 +2444,42 @@ void main() {
     }
     expect(() => tester.pumpWidget(buildApp()), throwsAssertionError);
   });
+
+  testWidgets('Skip the ScrollPosition check if the bar was unmounted', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/103939
+    final ScrollController scrollController = ScrollController();
+    Widget buildApp(bool buildBar) {
+      return Directionality(
+        textDirection: TextDirection.ltr,
+        child: MediaQuery(
+          data: MediaQueryData(
+            invertColors: buildBar, // Trigger a post frame check before unmount.
+          ),
+          child: PrimaryScrollController(
+            controller: scrollController,
+            child: LayoutBuilder(
+              builder: (BuildContext context, BoxConstraints constraints) {
+                Widget content = const SingleChildScrollView(
+                  child: SizedBox(width: 4000.0, height: 4000.0),
+                );
+                if (buildBar) {
+                  content = RawScrollbar(
+                    thumbVisibility: true,
+                    child: content,
+                  );
+                }
+                return content;
+              },
+            ),
+          ),
+        ),
+      );
+    }
+
+    await tester.pumpWidget(buildApp(true));
+
+    await tester.pumpWidget(buildApp(false));
+
+    // Go without throw.
+  });
 }