Add error message when sliver overlap absorber is missing (#128075)

Adds an error message where we previously would crash without any help.
Came across this while working on https://github.com/flutter/flutter/pull/127718
Fixes https://github.com/flutter/flutter/issues/128074

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

@@ -1914,6 +1914,15 @@ class RenderSliverOverlapInjector extends RenderSliver {
   void performLayout() {
     _currentLayoutExtent = handle.layoutExtent;
     _currentMaxExtent = handle.layoutExtent;
+    assert(
+      _currentLayoutExtent != null && _currentMaxExtent != null,
+      'SliverOverlapInjector has found no absorbed extent to inject.\n '
+      'The SliverOverlapAbsorber must be an earlier descendant of a common '
+      'ancestor Viewport, so that it will always be laid out before the '
+      'SliverOverlapInjector during a particular frame.\n '
+      'The SliverOverlapAbsorber is typically contained in the list of slivers '
+      'provided by NestedScrollView.headerSliverBuilder.\n'
+    );
     final double clampedLayoutExtent = math.min(
       _currentLayoutExtent! - constraints.scrollOffset,
       constraints.remainingPaintExtent,

packages/flutter/test/widgets/nested_scroll_view_test.dart

@@ -2908,6 +2908,46 @@ void main() {
     // Restructuring inner scrollable while scroll is in progress shouldn't crash.
     await tester.pumpWidget(buildApp(nested: true));
   });
+
+  testWidgets('SliverOverlapInjector asserts when there is no SliverOverlapAbsorber', (WidgetTester tester) async {
+    Widget buildApp() {
+      return MaterialApp(
+        home: Scaffold(
+          body: NestedScrollView(
+            headerSliverBuilder: (BuildContext context, bool innerBoxIsScrolled) {
+              return <Widget>[
+                const SliverAppBar(),
+              ];
+            },
+            body: Builder(
+              builder: (BuildContext context) {
+                return CustomScrollView(
+                  slivers: <Widget>[
+                    SliverOverlapInjector(
+                      handle: NestedScrollView.sliverOverlapAbsorberHandleFor(context),
+                    ),
+                  ],
+                );
+              }
+            ),
+          ),
+        ),
+      );
+    }
+    final List<Object> exceptions = <Object>[];
+    final FlutterExceptionHandler? oldHandler = FlutterError.onError;
+    FlutterError.onError = (FlutterErrorDetails details) {
+      exceptions.add(details.exception);
+    };
+    await tester.pumpWidget(buildApp());
+    FlutterError.onError = oldHandler;
+    expect(exceptions.length, 4);
+    expect(exceptions[0], isAssertionError);
+    expect(
+      (exceptions[0] as AssertionError).message,
+      contains('SliverOverlapInjector has found no absorbed extent to inject.'),
+    );
+  });
 }
 
 double appBarHeight(WidgetTester tester) => tester.getSize(find.byType(AppBar, skipOffstage: false)).height;