adds `isAttached` getter to DraggableScrollableController (#100269)

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

@@ -71,6 +71,14 @@ class DraggableScrollableController extends ChangeNotifier {
     return _attachedController!.extent.sizeToPixels(size);
   }
 
+  /// Returns Whether any [DraggableScrollableController] objects have attached themselves to the
+  /// [DraggableScrollableSheet].
+  ///
+  /// If this is false, then members that interact with the [ScrollPosition],
+  /// such as [sizeToPixels], [size], [animateTo], and [jumpTo], must not be
+  /// called.
+  bool get isAttached => _attachedController != null && _attachedController!.hasClients;
+
   /// Convert a sheet's pixel height to size (fractional value of parent container height).
   double pixelsToSize(double pixels) {
     _assertAttached();
@@ -157,7 +165,7 @@ class DraggableScrollableController extends ChangeNotifier {
 
   void _assertAttached() {
     assert(
-      _attachedController != null,
+      isAttached,
       'DraggableScrollableController is not attached to a sheet. A DraggableScrollableController '
         'must be used in a DraggableScrollableSheet before any of its methods are called.',
     );

packages/flutter/test/widgets/draggable_scrollable_sheet_test.dart

@@ -1195,4 +1195,18 @@ void main() {
     // Can't use animateTo with a zero duration.
     expect(() => controller.animateTo(.5, duration: Duration.zero, curve: Curves.linear), throwsAssertionError);
   });
+
+  testWidgets('DraggableScrollableController must be attached before using any of its paramters', (WidgetTester tester) async {
+    final DraggableScrollableController controller = DraggableScrollableController();
+    expect(controller.isAttached, false);
+    expect(()=>controller.size, throwsAssertionError);
+    final Widget boilerplate = _boilerplate(
+        null,
+        minChildSize: 0.4,
+        controller: controller,
+      );
+    await tester.pumpWidget(boilerplate);
+    expect(controller.isAttached, true);
+    expect(controller.size, isNotNull);
+    });
 }