Fix memory leak in _DraggableScrollableSheetState (#134212)

Author: ksokolovskyi

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

@@ -718,7 +718,11 @@ class _DraggableScrollableSheetState extends State<DraggableScrollableSheet> {
 
   @override
   void dispose() {
-    widget.controller?._detach(disposeExtent: true);
+    if (widget.controller == null) {
+      _extent.dispose();
+    } else {
+      widget.controller!._detach(disposeExtent: true);
+    }
     _scrollController.dispose();
     super.dispose();
   }

packages/flutter/test/material/about_test.dart

@@ -1590,12 +1590,7 @@ void main() {
     expect(titleOffset, const Offset(292.0, 136.0));
     expect(titleOffset.dx, lessThan(wideSize.width - 320)); // Default master view width is 320.0.
     expect(tester.getCenter(find.byType(ListView)), const Offset(160, 356));
-  },
-  leakTrackingTestConfig: const LeakTrackingTestConfig(
-    // TODO(polina-c): remove after fixing
-    // https://github.com/flutter/flutter/issues/133705
-    notDisposedAllowList: <String, int?> {'ValueNotifier<double>':5},
-  ));
+  });
 
   testWidgets('Material2 - LicensePage master view layout position - rtl', (WidgetTester tester) async {
     const TextDirection textDirection = TextDirection.rtl;