Testet with ZK 8.00 + latest 8.0.1.FL.20160106 by activation a mobile emulation in Chrome Debugger.
Decrease the width of the device screen and reload the page.

File: zul.tab.wpd

_showbutton : function(show) {
var tabbox = this.getTabbox();
if (tabbox.isTabscroll()) {
var cls = tabbox.$s('scroll');
jq(tabbox).removeClass(cls);
if (show) {

if(!tabbox.isVertical() && !tabbox.inAccordionMold())

{ var tb = tabbox.toolbar; tabbox.$n('left').style.height = tabbox.$n('right').style.height = ''; if (tb) tb.$n().style.height = ''; }

jq(tabbox).addClass(cls);
}
}
},

======>>>> tb.$n().style.height = ''; throws the error

-------------------------------------------------------
ncaught TypeError: Cannot read property 'style' of undefinedzul.tab.Tabs.zk.$extends.showbutton @ zul.tab.wpd:616zul.tab.Tabs.zk.$extends._scrollcheck @ zul.tab.wpd:530zul.tab.Tab.zk.$extends._setSel @ zul.tab.wpd:874zul.tab.Tab.zk.$extends._sel @ zul.tab.wpd:836(anonymous function) @ zul.tab.wpd:965doAfterMount @ zk.wpd:19073mtBL1 @ zk.wpd:19026mtBL0 @ zk.wpd:19018mtBL @ zk.wpd:18983zk.copy.zkx @ zk.wpd:19242zk.copy.zkmx @ zk.wpd:19270zkmx._childjs @ main.zul:86zul.wgt.Include.zk.$extends.bind @ zk.wpd:25948zk.Widget.zk.$extends.bindChildren_ @ zk.wpd:17043zk.Widget.zk.$extends.bind_ @ zk.wpd:17013zk.Object.$supers @ zk.wpd:10743zul.ContainerWidget.zk.$extends.bind_ @ zk.wpd:23604zk.Object.$supers @ zk.wpd:10743zul.tab.Tabpanel.zk.$extends.bind_ @ zul.tab.wpd:1332zk.Widget.zk.$extends.bindChildren_ @ zk.wpd:17043zk.Widget.zk.$extends.bind_ @ zk.wpd:17013zk.Object.$supers @ zk.wpd:10743zul.tab.Tabpanels.zk.$extends.bind_ @ zul.tab.wpd:1126zk.Widget.zk.$extends.bindChildren_ @ zk.wpd:17043zk.Widget.zk.$extends.bind_ @ zk.wpd:17013zk.Object.$supers @ zk.wpd:10743zul.tab.Tabbox.zk.$extends.bind_ @ zul.tab.wpd:124zk.Widget.zk.$extends.bindChildren_ @ zk.wpd:17043zk.Widget.zk.$extends.bind_ @ zk.wpd:17013zk.Object.$supers @ zk.wpd:10743zul.wgt.Include.zk.$extends.bind_ @ zk.wpd:25945zk.Widget.zk.$extends.bindChildren_ @ zk.wpd:17043zk.Widget.zk.$extends.bind_ @ zk.wpd:17013zk.Widget.zk.$extends.bindChildren_ @ zk.wpd:17043zk.Widget.zk.$extends.bind_ @ zk.wpd:17013zk.Object.$supers @ zk.wpd:10743zul.box.Layout.zk.$extends.bind_ @ zul.box.wpd:32zk.Widget.zk.$extends.bindChildren_ @ zk.wpd:17043zk.Widget.zk.$extends.bind_ @ zk.wpd:17013zk.Object.$supers @ zk.wpd:10743zul.layout.LayoutRegion.zk.$extends.bind_ @ zul.layout.wpd:669zk.Widget.zk.$extends.bindChildren_ @ zk.wpd:17043zk.Widget.zk.$extends.bind_ @ zk.wpd:17013zk.Object.$supers @ zk.wpd:10743zul.layout.Borderlayout.zk.$extends.bind_ @ zul.layout.wpd:95zk.Widget.zk.$extends.bindChildren_ @ zk.wpd:17043zk.Widget.zk.$extends.bind_ @ zk.wpd:17013zk.Object.$supers @ zk.wpd:10743zul.ContainerWidget.zk.$extends.bind_ @ zk.wpd:23604zk.Object.$supers @ zk.wpd:10743zul.wnd.Window.zk.$extends.bind_ @ zul.wnd.wpd:889zk.Widget.zk.$extends.bindChildren_ @ zk.wpd:17043zk.Widget.zk.$extends.bind_ @ zk.wpd:17013zk.Widget.zk.$extends.bind @ zk.wpd:16972zk.Widget.zk.$extends.replaceChildHTML_ @ zk.wpd:16804zk.Widget.zk.$extends.replaceHTML @ zk.wpd:16715mtBL0 @ zk.wpd:19012mtBL @ zk.wpd:18983doEnd @ zk.wpd:18593zk.copy.setLoaded._zkf @ zk.wpd:18631(anonymous function) @ zul.menu.wpd:1630doEnd @ zk.wpd:18593zk.copy.setLoaded._zkf @ zk.wpd:18630(anonymous function) @ zul.box.wpd:1719doEnd @ zk.wpd:18593zk.copy.setLoaded._zkf @ zk.wpd:18630(anonymous function) @ zul.layout.wpd:1700doEnd @ zk.wpd:18593zk.copy.setLoaded._zkf @ zk.wpd:18630(anonymous function) @ zul.wnd.wpd:2427doEnd @ zk.wpd:18593zk.copy.setLoaded._zkf @ zk.wpd:18630(anonymous function) @ zul.tab.wpd:1381doEnd @ zk.wpd:18593zk.copy.setLoaded._zkf @ zk.wpd:18630(anonymous function) @ zul.menu.wpd:1630
measureIt.js:120 Uncaught TypeError: Cannot read property 'create' of undefined(anonymous function) @ measureIt.js:120