To reproduce:

1. Download attached project "TestUI.zip" and open in Unity
2. Open "Test" scene
3. Delete Text component in the Canvas > Scroll View > Viewport > Content
4. Add new Text component to the Canvas > Scroll View > Viewport > Content
5. Press "Play" button

Expected result: After you pressed "Play" button, you will see, that no error popped up in "Console" window
Actual result: After you pressed "Play" button, you will see, that NullReferenceException: (null) error popped up in "Console" window

Note: Error disappears when you save project, restart Unity and reload the project

Reproduced on Unity 5.6.0p1 and 2017.1.0b1
Not reproduced on Unity 5.5.3p1
Regression on Unity 5.6.0b2