Steps to Reproduce

<zk>
  <checkbox id="cbox" tabindex="1"/>
  <button label="Add tabIndex" onClick="cbox.setTabindex(0)" />
  <button label="Remove tabIndex"  onClick="cbox.setTabindex(null)"/>
</zk>

Click both buttons, neither will fail to set.

Current Result

Error: Failed to process setAttr tabindex is not defined (ReferenceError)

Expected Result

Set tabIndex correctly.

Debug Information

https://github.com/zkoss/zk/blob/v8.0.2/zul/src/archive/web/js/zul/wgt/Checkbox.js#L96
Unresolved variable 'tabindex'

Workaround