Dynamically updating the preview and cover renderer causes issues

[flekschas]

It doesn't seem to be possible to dynamically change the preview and cover renderer and aggregators at the moment.

For testing please download the books example and go to http://localhost:8080/?example=books&options=1 At the bottom of the right sidebar there are 4 pile encoding options. The first two work but clicking on the third results in the following error:

item.js:41 Uncaught (in promise) TypeError: Cannot read property 'destroy' of null
    at createItem.replaceImage (item.js:41)
    at eval (library.js:1011)
    at Array.forEach (<anonymous>)
    at eval (library.js:1006)
    at async Promise.all (:8080/index 0)

A click on the fourth option just doesn't do anything. It works thought after you disperse and re-pile the items.

I suspect that we don't properly trigger the cover and preview rendering function