[Bug Report] when click the element of the last level twice, **@active-item-change** event doesn't work

[kingpriest]

Element UI version

2.4.4

OS/Browsers version

Chrome 67.0

Vue version

2.5.17-beta.0

Reproduction Link

https://jsfiddle.net/priestlau/vefkuso3/3/

Steps to reproduce

when click the element of the last level twice, @active-item-change event doesn't work

What is Expected?

the argument (val) of @active-item-change event function will be updated

What is actually happening?

when click the element of the last level twice, @active-item-change event doesn't work

click once

click twice