public List<DocRef> getChildren(final DocRef parent) {
        final String parentUuid = NullSafe.get(parent, DocRef::getUuid);
        if (parentKeyToChildNodesMap.containsKey(parentUuid)) {
            return parentKeyToChildNodesMap.get(parentUuid)
                    .stream()
                    .map(ExplorerNode::getDocRef)
                    .toList();
        } else {
            return null;
        }
    }
{@link AbstractTreeModel} keyed by doc UUID.
stroom-explorer/stroom-explorer-impl/src/main/java/stroom/explorer/impl/TreeModel.java