Steps to Reproduce

http://zkfiddle.org/sample/1bubpl7/1-Paging-Tree-clone-by-serialization-NPE

• click "Click me shouldn't have any error."

Current Result

NPE

Expected Result

No NPE

Debug Information

https://github.com/zkoss/zk/blob/v8.0.0/zul/src/org/zkoss/zul/AbstractTreeModel.java#L70
A transient _pagingListeners need to create a new instance when cloned.

Workaround

1. Set mold as default before cloning.
2. Just using org.zkoss.zk.ui.Component#clone