When prefabs are dragged across Scene view to place them to an empty script field in the Inspector, after placing the prefab, it stays in the Scene as a ghost object - there's no reference to it in the Hierarchy and it cannot be selected by clicking.

Steps to reproduce:
1) Open the attached project.
2) In the Hierarchy, select 'example' game object.
3) From the Assets folder in Project tab, drag the 'Cube' prefab so that it appears in the Scene while dragging it across.
4) Place the 'Cube' into the 'Example Object' field of 'example' game object in the Inspector.

After placing the 'Cube' on the script field, it stays in the Scene as well; it cannot be clicked on and doesn't appear in the Hierarchy. After restarting the Editor or reimporting the project, the cube starts acting normally, however it shouldn't have appeared in the first place.

Reproduced on:
5.6.0a1 (1e795b52556b), 5.5.0xb5, 5.4.1f1