[2D] [Sprite Atlas] New sprite gets added twice to Objects for Packing list

Steps to Repro:

1. create a new Sprite Atlas
2. click the + button to add a new Sprite to the Atlas
3. select a sprite for the assets then close the box

Result: The selected sprite gets added twice to the Objects for Packing list.
Expected: The selected sprite is only added once to the Objects for Packing list.