Steps to Reproduce

https://zkfiddle.org/sample/2cifl1i/1-Large-tree-with-treefoot-but-no-treecols-cause-JS-error

click the button to add rows

Current Result

Uncaught TypeError: Cannot read property 'cells' of undefined
    at _cpCellWd (zul.mesh.wpd:364)
    at init._calcSize (zul.mesh.wpd:1216)
    at init.onSize (zul.mesh.wpd:1109)
    at init.$supers (zk.wpd:12956)
    at init.onSize (zul.sel.wpd:3938)
    at init.onSize (zk.wpd:46082)
    at _invoke (zk.wpd:16465)
    at init.fire (zk.wpd:16448)
    at _fire (zk.wpd:16591)
    at Object.fireDown (zk.wpd:16808)

Expected Result

calculation doesn't cause error

Debug Information

https://github.com/zkoss/zk/blob/d050198077c1e285616e9d01771635ea48d5ad31/zul/src/archive/web/js/zul/mesh/MeshWidget.js#L353

srcrows contains loaded rows only, but wgt.getBodyWidgetIterator provide all items (including unloaded items).

_cpCellWd is only invoked when there is no treecol

https://github.com/zkoss/zk/blob/d050198077c1e285616e9d01771635ea48d5ad31/zul/src/archive/web/js/zul/mesh/MeshWidget.js#L1210-L1216

Workaround

Add an empty treecol to the tree

<treecols style="visibility: collapse"> <treecols style="visibility: collapse"> <treecol /> </treecols>