Null check

Bug: 1096515
Change-Id: I753e4d934b6c92d24f7b4276fc5fa9b716b45ffc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2252418
Auto-Submit: Aaron Leventhal <aleventhal@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#779946}

third_party/blink/renderer/modules/accessibility/ax_node_object.cc

@@ -171,7 +171,7 @@ AXObject* AXNodeObject::ActiveDescendant() {
     return nullptr;
 
   AXObject* ax_descendant = AXObjectCache().GetOrCreate(descendant);
-  return ax_descendant->IsVisible() ? ax_descendant : nullptr;
+  return ax_descendant && ax_descendant->IsVisible() ? ax_descendant : nullptr;
 }
 
 AXObjectInclusion AXNodeObject::ShouldIncludeBasedOnSemantics(