Can not move DraggableObject into SortableObjects if SortableObjects is empty

[Zordaxy]

When I setup multiple SortableObject containers and the starting state is for a container to be empty (no DraggableObjects moved into it yet), I cannot move a DraggableObject into that SortableObject container.
Similarly, if I move all of the DraggableObjects out of a SortableObject container, once the container is empty I can no longer move DraggableObjects back into the SortableObject container.
You can see this behavior in the demos, for example: http://mharris717.github.io/ember-drag-drop/#/sort
Move all of the Number Objects from one list to the other and you will not be able to move back into list once it is empty.

Similar issue in old repository: mharris717/ember-drag-drop#192