Remove nullOk from Scaffold.of and ScaffoldMessenger.of, create maybeOf for both (#68908)

packages/flutter/lib/src/material/app_bar.dart

@@ -487,11 +487,11 @@ class _AppBarState extends State<AppBar> {
   static const Color _defaultShadowColor = Color(0xFF000000);
 
   void _handleDrawerButton() {
-    Scaffold.of(context)!.openDrawer();
+    Scaffold.of(context).openDrawer();
   }
 
   void _handleDrawerButtonEnd() {
-    Scaffold.of(context)!.openEndDrawer();
+    Scaffold.of(context).openEndDrawer();
   }
 
   @override
@@ -500,7 +500,7 @@ class _AppBarState extends State<AppBar> {
     assert(debugCheckHasMaterialLocalizations(context));
     final ThemeData? theme = Theme.of(context);
     final AppBarTheme appBarTheme = AppBarTheme.of(context);
-    final ScaffoldState? scaffold = Scaffold.of(context, nullOk: true);
+    final ScaffoldState? scaffold = Scaffold.maybeOf(context);
     final ModalRoute<dynamic>? parentRoute = ModalRoute.of(context);
 
     final bool hasDrawer = scaffold?.hasDrawer ?? false;

packages/flutter/lib/src/material/bottom_sheet.dart

@@ -737,7 +737,7 @@ PersistentBottomSheetController<T> showBottomSheet<T>({
   assert(builder != null);
   assert(debugCheckHasScaffold(context));
 
-  return Scaffold.of(context)!.showBottomSheet<T>(
+  return Scaffold.of(context).showBottomSheet<T>(
     builder,
     backgroundColor: backgroundColor,
     elevation: elevation,

packages/flutter/lib/src/material/scaffold.dart

@@ -204,16 +204,34 @@ class ScaffoldMessenger extends StatefulWidget {
   /// ```
   /// {@end-tool}
   ///
-  /// If there is no [ScaffoldMessenger] in scope, then this will return null.
+  /// If there is no [ScaffoldMessenger] in scope, then this will assert in
+  /// debug mode, and throw an exception in release mode.
+  ///
   /// See also:
   ///
+  ///  * [maybeOf], which is a similar function but will return null instead of
+  ///    throwing if there is no [ScaffoldMessenger] ancestor.
   ///  * [debugCheckHasScaffoldMessenger], which asserts that the given context
   ///    has a [ScaffoldMessenger] ancestor.
-  static ScaffoldMessengerState? of(BuildContext context, { bool nullOk = false }) {
-    assert(nullOk != null);
+  static ScaffoldMessengerState of(BuildContext context) {
     assert(context != null);
+    assert(debugCheckHasScaffoldMessenger(context));
+
+    final _ScaffoldMessengerScope scope = context.dependOnInheritedWidgetOfExactType<_ScaffoldMessengerScope>()!;
+    return scope._scaffoldMessengerState;
+  }
 
-    assert(nullOk || debugCheckHasScaffoldMessenger(context));
+  /// The state from the closest instance of this class that encloses the given
+  /// context, if any.
+  ///
+  /// Will return null if a [ScaffoldMessenger] is not found in the given context.
+  ///
+  /// See also:
+  ///
+  ///  * [of], which is a similar function, except that it will throw an
+  ///    exception if a [ScaffoldMessenger] is not found in the given context.
+  static ScaffoldMessengerState? maybeOf(BuildContext context) {
+    assert(context != null);
 
     final _ScaffoldMessengerScope? scope = context.dependOnInheritedWidgetOfExactType<_ScaffoldMessengerScope>();
     return scope?._scaffoldMessengerState;
@@ -1755,7 +1773,11 @@ class Scaffold extends StatefulWidget {
   /// By default, the drag gesture is enabled.
   final bool endDrawerEnableOpenDragGesture;
 
-  /// The state from the closest instance of this class that encloses the given context.
+  /// Finds the [ScaffoldState] from the closest instance of this class that
+  /// encloses the given context.
+  ///
+  /// If no instance of this class encloses the given context, will cause an
+  /// assert in debug mode, and throw an exception in release mode.
   ///
   /// {@tool dartpad --template=freeform}
   /// Typical usage of the [Scaffold.of] function is to call it from within the
@@ -1892,12 +1914,11 @@ class Scaffold extends StatefulWidget {
   /// [ScaffoldState] rather than using the [Scaffold.of] function.
   ///
   /// If there is no [Scaffold] in scope, then this will throw an exception.
-  /// To return null if there is no [Scaffold], then pass `nullOk: true`.
-  static ScaffoldState? of(BuildContext context, { bool nullOk = false }) {
-    assert(nullOk != null);
+  /// To return null if there is no [Scaffold], use [maybeOf] instead.
+  static ScaffoldState of(BuildContext context) {
     assert(context != null);
     final ScaffoldState? result = context.findAncestorStateOfType<ScaffoldState>();
-    if (nullOk || result != null)
+    if (result != null)
       return result;
     throw FlutterError.fromParts(<DiagnosticsNode>[
       ErrorSummary(
@@ -1927,6 +1948,22 @@ class Scaffold extends StatefulWidget {
     ]);
   }
 
+  /// Finds the [ScaffoldState] from the closest instance of this class that
+  /// encloses the given context.
+  ///
+  /// If no instance of this class encloses the given context, will return null.
+  /// To throw an exception instead, use [of] instead of this function.
+  ///
+  /// See also:
+  ///
+  ///  * [of], a similar function to this one that throws if no instance
+  ///    encloses the given context. Also includes some sample code in its
+  ///    documentation.
+  static ScaffoldState? maybeOf(BuildContext context) {
+    assert(context != null);
+    return context.findAncestorStateOfType<ScaffoldState>();
+  }
+
   /// Returns a [ValueListenable] for the [ScaffoldGeometry] for the closest
   /// [Scaffold] ancestor of the given context.
   ///
@@ -2714,7 +2751,7 @@ class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin {
   void didChangeDependencies() {
     // nullOk is valid here since  both the Scaffold and ScaffoldMessenger are
     // currently available for managing SnackBars.
-    final ScaffoldMessengerState? _currentScaffoldMessenger = ScaffoldMessenger.of(context, nullOk: true);
+    final ScaffoldMessengerState? _currentScaffoldMessenger = ScaffoldMessenger.maybeOf(context);
     // If our ScaffoldMessenger has changed, unregister with the old one first.
     if (_scaffoldMessenger != null &&
       (_currentScaffoldMessenger == null || _scaffoldMessenger != _currentScaffoldMessenger)) {
@@ -3304,7 +3341,7 @@ class _StandardBottomSheetState extends State<_StandardBottomSheet> {
 
   bool extentChanged(DraggableScrollableNotification notification) {
     final double extentRemaining = 1.0 - notification.extent;
-    final ScaffoldState scaffold = Scaffold.of(context)!;
+    final ScaffoldState scaffold = Scaffold.of(context);
     if (extentRemaining < _kBottomSheetDominatesPercentage) {
       scaffold._floatingActionButtonVisibilityValue = extentRemaining * _kBottomSheetDominatesPercentage * 10;
       scaffold.showBodyScrim(true,  math.max(

packages/flutter/lib/src/material/snack_bar.dart

@@ -123,7 +123,7 @@ class _SnackBarActionState extends State<SnackBarAction> {
       _haveTriggeredAction = true;
     });
     widget.onPressed();
-    Scaffold.of(context)!.hideCurrentSnackBar(reason: SnackBarClosedReason.action);
+    Scaffold.of(context).hideCurrentSnackBar(reason: SnackBarClosedReason.action);
   }
 
   @override
@@ -529,14 +529,14 @@ class _SnackBarState extends State<SnackBar> {
       container: true,
       liveRegion: true,
       onDismiss: () {
-        Scaffold.of(context)!.removeCurrentSnackBar(reason: SnackBarClosedReason.dismiss);
+        Scaffold.of(context).removeCurrentSnackBar(reason: SnackBarClosedReason.dismiss);
       },
       child: Dismissible(
         key: const Key('dismissible'),
         direction: DismissDirection.down,
         resizeDuration: null,
         onDismissed: (DismissDirection direction) {
-          Scaffold.of(context)!.removeCurrentSnackBar(reason: SnackBarClosedReason.swipe);
+          Scaffold.of(context).removeCurrentSnackBar(reason: SnackBarClosedReason.swipe);
         },
         child: snackBar,
       ),

packages/flutter/test/material/floating_action_button_location_test.dart

@@ -647,7 +647,7 @@ void main() {
               builder: (BuildContext context) {
                 return FloatingActionButton(
                   onPressed: () {
-                    ScaffoldMessenger.of(context)!.showSnackBar(
+                    ScaffoldMessenger.of(context).showSnackBar(
                       const SnackBar(content: Text('Snacky!')),
                     );
                   },

packages/flutter/test/material/scaffold_test.dart

@@ -355,7 +355,7 @@ void main() {
             builder: (BuildContext context) {
               return GestureDetector(
                 onTap: () {
-                  Scaffold.of(context)!.showBottomSheet<void>((BuildContext context) {
+                  Scaffold.of(context).showBottomSheet<void>((BuildContext context) {
                     return Container(
                       key: sheetKey,
                       color: Colors.blue[500],
@@ -1900,7 +1900,7 @@ void main() {
         MaterialApp(
           home: Builder(
             builder: (BuildContext context) {
-              Scaffold.of(context)!.showBottomSheet<void>((BuildContext context) {
+              Scaffold.of(context).showBottomSheet<void>((BuildContext context) {
                 return Container();
               });
               return Container();
@@ -2033,7 +2033,7 @@ void main() {
     });
   });
 
-  testWidgets('ScaffoldMessenger.of can return null', (WidgetTester tester) async {
+  testWidgets('ScaffoldMessenger.maybeOf can return null if not found', (WidgetTester tester) async {
     ScaffoldMessengerState? scaffoldMessenger;
     const Key tapTarget = Key('tap-target');
     await tester.pumpWidget(Directionality(
@@ -2045,7 +2045,7 @@ void main() {
             builder: (BuildContext context) {
               return GestureDetector(
                 onTap: () {
-                  scaffoldMessenger = ScaffoldMessenger.of(context, nullOk: true);
+                  scaffoldMessenger = ScaffoldMessenger.maybeOf(context);
                 },
                 behavior: HitTestBehavior.opaque,
                 child: Container(
@@ -2064,7 +2064,7 @@ void main() {
     expect(scaffoldMessenger, isNull);
   });
 
-  testWidgets('ScaffoldMessenger.of will assert if !nullOk', (WidgetTester tester) async {
+  testWidgets('ScaffoldMessenger.of will assert if not found', (WidgetTester tester) async {
     const Key tapTarget = Key('tap-target');
 
     final List<dynamic> exceptions = <dynamic>[];

packages/flutter/test/material/snack_bar_theme_test.dart

@@ -71,7 +71,7 @@ void main() {
           builder: (BuildContext context) {
             return GestureDetector(
               onTap: () {
-                ScaffoldMessenger.of(context)!.showSnackBar(SnackBar(
+                ScaffoldMessenger.of(context).showSnackBar(SnackBar(
                   content: const Text(text),
                   duration: const Duration(seconds: 2),
                   action: SnackBarAction(label: 'ACTION', onPressed: () {}),
@@ -109,7 +109,7 @@ void main() {
             builder: (BuildContext context) {
               return GestureDetector(
                 onTap: () {
-                  ScaffoldMessenger.of(context)!.showSnackBar(SnackBar(
+                  ScaffoldMessenger.of(context).showSnackBar(SnackBar(
                     content: const Text(text),
                     duration: const Duration(seconds: 2),
                     action: SnackBarAction(label: action, onPressed: () {}),
@@ -153,7 +153,7 @@ void main() {
             builder: (BuildContext context) {
               return GestureDetector(
                 onTap: () {
-                  ScaffoldMessenger.of(context)!.showSnackBar(SnackBar(
+                  ScaffoldMessenger.of(context).showSnackBar(SnackBar(
                     backgroundColor: backgroundColor,
                     elevation: elevation,
                     shape: shape,
@@ -200,7 +200,7 @@ void main() {
           builder: (BuildContext context) {
             return GestureDetector(
               onTap: () {
-                ScaffoldMessenger.of(context)!.showSnackBar(SnackBar(
+                ScaffoldMessenger.of(context).showSnackBar(SnackBar(
                   content: const Text('I am a snack bar.'),
                   duration: const Duration(seconds: 2),
                   action: SnackBarAction(label: 'ACTION', onPressed: () {}),
@@ -242,7 +242,7 @@ void main() {
           builder: (BuildContext context) {
             return GestureDetector(
               onTap: () {
-                ScaffoldMessenger.of(context)!.showSnackBar(SnackBar(
+                ScaffoldMessenger.of(context).showSnackBar(SnackBar(
                   content: const Text('I am a snack bar.'),
                   duration: const Duration(seconds: 2),
                   action: SnackBarAction(label: 'ACTION', onPressed: () {}),