AX: AXCoreObject::enumerateDescendants is missing a null check on obj…

…ect children

https://bugs.webkit.org/show_bug.cgi?id=259138
rdar://100626906

Reviewed by Chris Fleizach.

Prevent nullptr crashes by checking that each child from
AXCoreObject::children() is not null in AXCoreObject::enumerateDescendants.

In a future patch, we should make `AccessibilityChildrenVector` be a Vector of `Ref`s
rather than a Vector of `RefPtr`s as it is today. RefPtr does not make sense for this
type -- either we have a child or we don't.

* Source/WebCore/accessibility/AccessibilityObjectInterface.h:
(WebCore::Accessibility::enumerateDescendants):

Canonical link: https://commits.webkit.org/265992@main

Source/WebCore/accessibility/AccessibilityObjectInterface.h

@@ -1935,8 +1935,10 @@ void enumerateDescendants(T& object, bool includeSelf, const F& lambda)
     if (includeSelf)
         lambda(object);
 
-    for (const auto& child : object.children())
-        enumerateDescendants(*child, true, lambda);
+    for (const auto& child : object.children()) {
+        if (child)
+            enumerateDescendants(*child, true, lambda);
+    }
 }
 
 template<typename U> inline void performFunctionOnMainThreadAndWait(U&& lambda)