fix(runtime-core): catch undefined element

[jaulz]

Right now, when the element of leavingVNode is null it will fail and throw an exception. This PR will only call the callback if the element is set.

[posva]

Do you have a boiled down reproduction or a failing test case?

[jaulz]

Hm, I tried the whole morning to create a reproducible simple test case but for some reason it only appears in my "big" app. Also, as soon as I set breakpoint in one of the triggers (that I can see in the stacktrace) the issue does not appear anymore.

The issue appears when I leave the tab and activate it again. I think in that case the tab loses focus first and then immediately gains focus again.

I am not 100% familiar with the core of the framework but I see that there are two effects queued so maybe there is some concurrency issue:

The last patch looks like this (n1 is null in this case):

Afterwards it runs into the beforeEnter of the transition and for some reason the el attribute is empty both of vnode and of leavingVNode:

But irrespectively of this issue, shouldn't the exclamation mark anyway be handled differently since it's not really best practise?

[yyx990803]

The reason it is using ! here is because by the logic of BaseTranstion, only mounted vnodes could ever be saved in the leavingVNodes cache. So if vnodes in the cache has null el values, there's something wrong elsewhere. (Which is why we want a reproduction so we can fix the actual root cause)

[ferferga]

@yyx990803 @posva Here's a simple reproduction

As soon as class is removed from BTransition, the problem is fixed.

[edison1105]

duplicate of #10497
the root cause #3716 (comment)