fix: 'PageStorage.of' returns null (AppFlowy-IO#575)

lib/src/flutter/scrollable_positioned_list/src/scrollable_positioned_list.dart

@@ -339,7 +339,8 @@ class _ScrollablePositionedListState extends State<ScrollablePositionedList>
   @override
   void initState() {
     super.initState();
-    ItemPosition? initialPosition = PageStorage.of(context).readState(context);
+    ItemPosition? initialPosition =
+        PageStorage.maybeOf(context)?.readState(context);
     primary.target = initialPosition?.index ?? widget.initialScrollIndex;
     primary.alignment =
         initialPosition?.itemLeadingEdge ?? widget.initialAlignment;
@@ -670,7 +671,7 @@ class _ScrollablePositionedListState extends State<ScrollablePositionedList>
           position.itemLeadingEdge < 1 && position.itemTrailingEdge > 0,
     );
     if (itemPositions.isNotEmpty) {
-      PageStorage.of(context).writeState(
+      PageStorage.maybeOf(context)?.writeState(
         context,
         itemPositions.reduce(
           (value, element) =>

test/customer/editor_on_overlay_test.dart

@@ -0,0 +1,73 @@
+import 'package:appflowy_editor/appflowy_editor.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() async {
+  setUpAll(() {
+    TestWidgetsFlutterBinding.ensureInitialized();
+  });
+
+  testWidgets('customize highlight color', (tester) async {
+    await tester.pumpWidget(const MyApp());
+    await tester.pumpAndSettle();
+
+    // tap the open editor button
+    final openEditorButton = find.byType(FloatingActionButton);
+    await tester.tap(openEditorButton);
+    await tester.pumpAndSettle();
+  });
+}
+
+class MyApp extends StatelessWidget {
+  const MyApp({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      title: 'Flutter Demo',
+      theme: ThemeData(
+        colorScheme: ColorScheme.fromSeed(seedColor: Colors.deepPurple),
+        useMaterial3: true,
+      ),
+      home: const MyHomePage(title: 'Flutter Demo Home Page'),
+    );
+  }
+}
+
+class MyHomePage extends StatefulWidget {
+  const MyHomePage({super.key, required this.title});
+
+  final String title;
+
+  @override
+  State<MyHomePage> createState() => _MyHomePageState();
+}
+
+class _MyHomePageState extends State<MyHomePage> {
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(
+        backgroundColor: Theme.of(context).colorScheme.inversePrimary,
+        title: Text(widget.title),
+      ),
+      body: const Placeholder(),
+      floatingActionButton: FloatingActionButton(
+        onPressed: _insertEditorOnOverlay,
+        child: const Icon(Icons.add),
+      ),
+    );
+  }
+
+  void _insertEditorOnOverlay() {
+    final overlayEntry = OverlayEntry(
+      builder: (_) {
+        return AppFlowyEditor(
+          editorState: EditorState.blank(),
+        );
+      },
+    );
+
+    Overlay.of(context).insert(overlayEntry);
+  }
+}