fix: set sortableGhost to null when sort ended

[DanSnow]

Click on an item in a list with group + press-delay will trigger the drag canceled which will clean up the sortableGhost by removing it. But the sortableGhost is also used for setting it to invisible when sorted in place. So in the drag canceled handler will remove the item that we just sorted.

Fixed #161

[Knogobert]

Works nicely! Thanks. Let's hope this gets merged soon 🌟