listbox select mold produces js error in a mobile browser

    <listbox mold="select">
        <listgroup label="2018"/>
        <listitem label="2018 Q1"/>
        <listitem label="2018 Q2"/>
        <listitem label="2018 Q3"/>
        <listitem label="2018 Q4"/>
        <listgroup label="2017"/>
        <listitem label="2017 Q1"/>
        <listitem label="2017 Q2"/>
        <listitem label="2017 Q3"/>
        <listitem label="2017 Q4"/>
    </listbox>

zk.override(zk.Widget.prototype, _xWidget, {
	setMold: zk.$void // not support different mold for tablet devices
});