Bug 1438326. Don't try to enqueue custom element callbacks when the custom element definition has been unlinked. r=smaug

MozReview-Commit-ID: FLf6CJcpcVQ

dom/base/CustomElementRegistry.cpp

@@ -423,6 +423,11 @@ CustomElementRegistry::EnqueueLifecycleCallback(nsIDocument::ElementCallbackType
         definition->mLocalName != aCustomElement->NodeInfo()->NameAtom()) {
       return;
     }
+
+    if (!definition->mCallbacks) {
+      // definition has been unlinked.  Don't try to mess with it.
+      return;
+    }
   }
 
   auto callback =