Event Occurs After Handler is Deleted

[rmorshea]

Consider a case where a button click causes that button to be re-rendered. In this scenario, after the first click the backend will begin work to render again and eventually will delete state for the original button element. If shortly after the first button click a user clicks again it is possible that the button in the view and the button model in the backend are not the same. The event produced by this last click will result in an error because the element that would normally have handled it has already been deleted.

In particular we get a key error here:

https://github.com/rmorshea/idom/blob/b21f8e8361ced41508fb80afab4d5efe69c423fe/src/py/idom/layout.py#L60