nativebar=false causes "after mounting error"

Steps to Reproduce

1. set library property org.zkoss.zul.nativebar with false
2. run the zul below

    <listbox>
        <listhead>
            <listheader label="Client" hflex="min"/>
        </listhead>
        <listitem>
            <listcell label="aaa" hflex="min"/>
        </listitem>
    </listbox>

Current Result

Uncaught TypeError: Cannot read property 'firstChild' of null
at _calcMinWd (zul.mesh.wpd:62)
at init._calcMinWds (zul.mesh.wpd:1430)
at init.beforeMinFlex_ (zul.mesh.wpd:2917)
at Object.fixMinFlex (zk.wpd:27771)
at Object.fixFlex (zk.wpd:27661)
at init.onSize (zk.wpd:27566)
at _invoke (zk.wpd:15020)
at init.fire (zk.wpd:15003)
at _fire (zk.wpd:15146)
at Object.fireDown (zk.wpd:15363)

Then show

Error code 3: ZK error, after mounting.

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Expected Result

no error.

Debug Information

it seems non-nativebar usually causes problems, do we consider to deprecate this property?

Workaround

set nativebar to true.