Cherry-pick 46a7bd3. rdar://124421166

    AX: com.apple.WebKit.WebContent crash at WebCore: -[WebAccessibilityObjectWrapper handleNotificationRelayToChrome:notificationData:]
    https://bugs.webkit.org/show_bug.cgi?id=269319
    <rdar://problem/122688370>

    Reviewed by (OOPS).

    Ensure that we have a valid AXObjectCache before proceeding.

    * Source/WebCore/accessibility/ios/WebAccessibilityObjectWrapperIOS.mm

    Canonical link: https://commits.webkit.org/274610@main

Source/WebCore/accessibility/ios/WebAccessibilityObjectWrapperIOS.mm

@@ -1711,7 +1711,8 @@ - (void)handleNotificationRelayToChrome:(NSString *)notificationName notificatio
     if (![self _prepareAccessibilityCall])
         return;
 
-    self.axBackingObject->axObjectCache()->relayNotification({ notificationName }, notificationData);
+    if (auto* cache = self.axBackingObject->axObjectCache())
+        cache->relayNotification({ notificationName }, notificationData);
 }
 
 - (CGRect)_accessibilityRelativeFrame