Fix return type of ContextAction.invoke (#70397)

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

@@ -342,7 +342,7 @@ abstract class ContextAction<T extends Intent> extends Action<T> {
   /// ```
   @protected
   @override
-  Object invoke(covariant T intent, [BuildContext? context]);
+  Object? invoke(covariant T intent, [BuildContext? context]);
 }
 
 /// The signature of a callback accepted by [CallbackAction].

packages/flutter/test/widgets/actions_test.dart

@@ -448,6 +448,33 @@ void main() {
       expect(identical(result, sentinel), isTrue);
       expect(invoked, isTrue);
     });
+
+    testWidgets('ContextAction can return null', (WidgetTester tester) async {
+      final GlobalKey containerKey = GlobalKey();
+      const TestIntent intent = TestIntent();
+      final TestContextAction testAction = TestContextAction();
+
+      await tester.pumpWidget(
+        Actions(
+          dispatcher: TestDispatcher1(postInvoke: collect),
+          actions: <Type, Action<Intent>>{
+            TestIntent: testAction,
+          },
+          child: Container(key: containerKey),
+        ),
+      );
+
+      await tester.pump();
+      final Object? result = Actions.invoke<TestIntent>(
+        containerKey.currentContext!,
+        intent,
+      );
+      expect(result, isNull);
+      expect(invokedIntent, equals(intent));
+      expect(invokedAction, equals(testAction));
+      expect(invokedDispatcher.runtimeType, equals(TestDispatcher1));
+      expect(testAction.capturedContexts.single, containerKey.currentContext);
+    });
   });
 
   group('Listening', () {
@@ -834,3 +861,13 @@ void main() {
     });
   });
 }
+
+class TestContextAction extends ContextAction<TestIntent> {
+  List<BuildContext?> capturedContexts = <BuildContext?>[];
+
+  @override
+  Object? invoke(covariant TestIntent intent, [BuildContext? context]) {
+    capturedContexts.add(context);
+    return null;
+  }
+}