Null Reference exception occurs when trying to add/delete the event or changing sample number.

Steps to reproduce:

1. Create a new Project or use the existing one.
2. Create a game object and open Animation window.
3. Select any game object and try to change the numbers in the sample.(Observe the errors in console).
4. Click on the add event and observe the errors in console.
5. Right-click on the keyFrame and try to add/delete the event from there and observe the error in the console.
6. Click on the keyframes and try to add the function from the inspector window and observe the error in the console.

Reproducible on : 2018.1.0b4, 2018.1.0b3, 2018.1.0b2, 2018.1.0b1, 2018.2.0a1(008bea5b6776)

Working Fine on: 2018.1.0a7, 2017.3.0f3

Regeressed Version: 2018.1.0b1

Environment: Occurring on both Mac and Windows