[Bug] Item drop duplicates after opening Layers Panel.

@artf After upgrading to 0.14.48, it is throwing an error while loading data from remote source as well dragging new elements on canvas.

Uncaught TypeError: s is not a constructor
    at r.addToCollection (grapes.js:39597)
    at r.addTo (grapes.js:39576)
    at v (grapes.js:4743)
    at m (grapes.js:4728)
    at d (grapes.js:4527)
    at r.c.trigger (grapes.js:4718)
    at r._onModelEvent (grapes.js:5559)
    at v (grapes.js:4744)
    at m (grapes.js:4729)
    at d (grapes.js:4527)

And,

grapes.js:53487 Uncaught TypeError: Cannot read property 'index' of undefined
    at r.move (grapes.js:53487)
    at grapes.js:53454
    at Array.forEach (<anonymous>)
    at r.endMove (grapes.js:53453)
    at I (grapes.js:21273)
    at HTMLDocument.<anonymous> (grapes.js:21286)
    at HTMLDocument.<anonymous> (grapes.js:20633)

https://github.com/artf/grapesjs/releases/tag/v0.14.49

Ok thanks guys, I'll publish soon a new release with this hotfix