I'm porting the #Beast web UI from Vue-2 to #Vue-3. Turns out to be more involved than I originally anticipated.

I kept getting an odd exception during my tests. It looks like Vue-3.0.0 can become confused when state watchers (created via Vue.$watch) are unregistered during a later stage of a component lifecycle - trying to remove item from `null`.

I fail to see the error in my code at least (which works fine in Vue-2) and suspect it's a genuine Vue bug, filed here:

https://github.com/vuejs/vue-next/issues/2381