Steps to reproduce:
1. Create empty project
2. Create Canvas element
3. Add Dropdown menu element to Canvas element
4. Play scene
5. Open and close Dropdown menu

Actual result: Cannot open the menu second time, error is shown in console:
"MissingReferenceException: The object of type 'Image' has been destroyed but you are still trying to access it."
Expected result: Dropdown menu opens and closes as expected everytime, not only just once

Tested and reproduced on:
Windows 10
Unity 5.4.0b5

Not reproducible on:
Unity 5.2.4p1, 5.3.0f4, 5.3.2p2