Fix memory leak of FlutterCustomAccessibilityAction (#17954)

diff --git a/shell/platform/darwin/ios/framework/Source/accessibility_bridge.mm b/shell/platform/darwin/ios/framework/Source/accessibility_bridge.mm
index c9c147b..00055c9 100644
--- a/shell/platform/darwin/ios/framework/Source/accessibility_bridge.mm
+++ b/shell/platform/darwin/ios/framework/Source/accessibility_bridge.mm
@@ -76,9 +76,9 @@
         NSString* label = @(action.label.data());
         SEL selector = @selector(onCustomAccessibilityAction:);
         FlutterCustomAccessibilityAction* customAction =
-            [[FlutterCustomAccessibilityAction alloc] initWithName:label
-                                                            target:object
-                                                          selector:selector];
+            [[[FlutterCustomAccessibilityAction alloc] initWithName:label
+                                                             target:object
+                                                           selector:selector] autorelease];
         customAction.uid = action_id;
         [accessibilityCustomActions addObject:customAction];
       }