To reproduce:

1.Create new Unit project.
2.Create Dropdown UI element.
3.Change it's Image component's Source image property from UISprite to Knob.
4.Enter Play mode.
5.Exit Play mode.
6.Delete Dropdown element from the Hierarchy.
7.Enter Play mode.

Expected result: No error in the console being thrown.
Actual result: NullReferenceException: (null) error is thrown in the console.

Reproduced with: 5.6.0b5, 5.6.0b6
Not reproduced with: 5.5.0p4, 5.4.4f1

Notes: Exception is thrown when Source image property is changed to any other than UISprite. When project is re-loaded error disappears.