How to reproduce:
1. Create new empty project
2. Start Collab
3. Create UI -> Button
4. Add new event subscription on OnClick()
5. Click on the GameObject field's circle

Expected result: There is should be no exceptions
Actual result: Popup window opens and AgrumentNullException appears on the console until you choose a GameObject from the popup window

Reproduced with: 2017.1.1f1, 2017.2.0b9, 2017.3.0a6
Not reproduced with: 5.6.3p2
Regression since: 2017.1.0b3

Note: Collab should be enabled