Collapsing a Groupbox with invalidate causes a JavaScript error

Steps to Reproduce

1. collapse groupbox

   <zk>
       <groupbox title="Groupbox"
                 onOpen="self.invalidate();">
           <button label="test button" />
       </groupbox>
   </zk>
   

Current Result

An error in browser console

TypeError: btn is undefined

Expected Result

No error in browser console

Debug Information

It tried to restore the focus, but the cave node is empty in collapsed groupbox.
Button isn't aware this and use this.$n() (=null)
https://github.com/zkoss/zk/blob/v8.6.0.1/zul/src/archive/web/js/zul/wgt/Button.js#L231

Workaround

Avoid calling invalidate if an animation is included in a action.
Use <caption> instead of title attribute. (Because you clicked caption, not groupbox itself)