Remove nullOk parameter from AnimatedList.of and SliverAnimatedList.of (#68925)

This removes the nullOk parameter from AnimatedList.of and SliverAnimatedList.of, and creates maybeOf equivalents for each. The of methods now return non-nullable values, and the maybeOf equivalents return nullable values.

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

@@ -382,18 +382,23 @@ class AnimatedList extends StatefulWidget {
   /// This method is typically used by [AnimatedList] item widgets that insert
   /// or remove items in response to user input.
   ///
-  /// ```dart
-  /// AnimatedListState animatedList = AnimatedList.of(context);
-  /// ```
-  static AnimatedListState? of(BuildContext context, { bool nullOk = false }) {
+  /// If no [AnimatedList] surrounds the context given, then this function will
+  /// assert in debug mode and throw an exception in release mode.
+  ///
+  /// See also:
+  ///
+  ///  * [maybeOf], a similar function that will return null if no
+  ///    [AnimatedList] ancestor is found.
+  static AnimatedListState of(BuildContext context) {
     assert(context != null);
-    assert(nullOk != null);
     final AnimatedListState? result = context.findAncestorStateOfType<AnimatedListState>();
-    if (nullOk || result != null)
-      return result;
+    assert((){
+      if (result == null) {
         throw FlutterError.fromParts(<DiagnosticsNode>[
-      ErrorSummary('AnimatedList.of() called with a context that does not contain an AnimatedList.'),
-      ErrorDescription('No AnimatedList ancestor could be found starting from the context that was passed to AnimatedList.of().'),
+          ErrorSummary(
+            'AnimatedList.of() called with a context that does not contain an AnimatedList.'),
+          ErrorDescription(
+            'No AnimatedList ancestor could be found starting from the context that was passed to AnimatedList.of().'),
           ErrorHint(
             'This can happen when the context provided is from the same StatefulWidget that '
             'built the AnimatedList. Please see the AnimatedList documentation for examples '
@@ -403,6 +408,28 @@ class AnimatedList extends StatefulWidget {
           context.describeElement('The context used was')
         ]);
       }
+      return true;
+    }());
+    return result!;
+  }
+
+  /// The state from the closest instance of this class that encloses the given
+  /// context.
+  ///
+  /// This method is typically used by [AnimatedList] item widgets that insert
+  /// or remove items in response to user input.
+  ///
+  /// If no [AnimatedList] surrounds the context given, then this function will
+  /// return null.
+  ///
+  /// See also:
+  ///
+  ///  * [of], a similar function that will throw if no [AnimatedList] ancestor
+  ///    is found.
+  static AnimatedListState? maybeOf(BuildContext context) {
+    assert(context != null);
+    return context.findAncestorStateOfType<AnimatedListState>();
+  }
 
   @override
   AnimatedListState createState() => AnimatedListState();
@@ -770,15 +797,18 @@ class SliverAnimatedList extends StatefulWidget {
   /// This method is typically used by [SliverAnimatedList] item widgets that
   /// insert or remove items in response to user input.
   ///
-  /// ```dart
-  /// SliverAnimatedListState animatedList = SliverAnimatedList.of(context);
-  /// ```
-  static SliverAnimatedListState? of(BuildContext context, {bool nullOk = false}) {
+  /// If no [SliverAnimatedList] surrounds the context given, then this function
+  /// will assert in debug mode and throw an exception in release mode.
+  ///
+  /// See also:
+  ///
+  ///  * [maybeOf], a similar function that will return null if no
+  ///    [SliverAnimatedList] ancestor is found.
+  static SliverAnimatedListState of(BuildContext context) {
     assert(context != null);
-    assert(nullOk != null);
     final SliverAnimatedListState? result = context.findAncestorStateOfType<SliverAnimatedListState>();
-    if (nullOk || result != null)
-      return result;
+    assert((){
+      if (result == null) {
         throw FlutterError(
           'SliverAnimatedList.of() called with a context that does not contain a SliverAnimatedList.\n'
           'No SliverAnimatedListState ancestor could be found starting from the '
@@ -786,10 +816,32 @@ class SliverAnimatedList extends StatefulWidget {
           'happen when the context provided is from the same StatefulWidget that '
           'built the AnimatedList. Please see the SliverAnimatedList documentation '
           'for examples of how to refer to an AnimatedListState object: '
-        'https://docs.flutter.io/flutter/widgets/SliverAnimatedListState-class.html \n'
+          'https://docs.flutter.io/flutter/widgets/SliverAnimatedListState-class.html\n'
           'The context used was:\n'
           '  $context');
       }
+      return true;
+    }());
+    return result!;
+  }
+
+  /// The state from the closest instance of this class that encloses the given
+  /// context.
+  ///
+  /// This method is typically used by [SliverAnimatedList] item widgets that
+  /// insert or remove items in response to user input.
+  ///
+  /// If no [SliverAnimatedList] surrounds the context given, then this function
+  /// will return null.
+  ///
+  /// See also:
+  ///
+  ///  * [of], a similar function that will throw if no [SliverAnimatedList]
+  ///    ancestor is found.
+  static SliverAnimatedListState? maybeOf(BuildContext context) {
+    assert(context != null);
+    return context.findAncestorStateOfType<SliverAnimatedListState>();
+  }
 }
 
 /// The state for a sliver that animates items when they are

packages/flutter/test/widgets/animated_list_test.dart

@@ -321,11 +321,12 @@ void main() {
     });
   });
 
-  testWidgets('AnimatedList.of() called with a context that does not contain AnimatedList',
+  testWidgets('AnimatedList.of() and maybeOf called with a context that does not contain AnimatedList',
     (WidgetTester tester) async {
     final GlobalKey key = GlobalKey();
     await tester.pumpWidget(Container(key: key));
     late FlutterError error;
+    expect(AnimatedList.maybeOf(key.currentContext!), isNull);
     try {
       AnimatedList.of(key.currentContext!);
     } on FlutterError catch (e) {