Fix crash when adding a child while duplicating a node

scene/main/node.cpp

@@ -2930,8 +2930,10 @@ void Node::_duplicate_properties_node(const Node *p_root, const Node *p_original
 		}
 	}
 
-	for (int i = 0; i < p_copy->get_child_count(); i++) {
-		_duplicate_properties_node(p_root, p_original->get_child(i), p_copy->get_child(i));
+	for (int i = 0; i < p_original->get_child_count(); i++) {
+		Node *copy_child = p_copy->get_child(i);
+		ERR_FAIL_NULL_MSG(copy_child, "Child node disappeared while duplicating.");
+		_duplicate_properties_node(p_root, p_original->get_child(i), copy_child);
 	}
 }