i've got an error with the roll a ball tutorial

the error is:
NullReferenceException: Object reference not set to an instance of an object
PlayerController.FixedUpdate () (at Assets/scripts/PlayerController.cs:20)

and this is my script:
59876-error.png

The gameobject that this script is attached to needs to have a rigidbody component attached as well. It looks like you may have forgotten to add a rigidbody to the object?