Have you browsed through the example scenes? There are scripts there that walk through the most basic animation controlling code.
In your code, GetComponent<SkeletonAnimation>()
may return null
if it failed to get the SkeletonAnimation
component.
A NullReferenceException gets thrown when you try to do anything with a null reference.
You got null likely because the GameObject you attached your ZombieMovement script to doesn't have a SkeletonAnimation component on it.
If you are using SkeletonAnimator instead, you need to interface with Mecanim to control the animation, not Spine's AnimationState. If you had a SkeletonAnimator instead of SkeletonAnimation, you would also get a null from GetComponent<SkeletonAnimation> since SkeletonAnimator is not SkeletonAnimation.
If you are new to Unity and Spine, we recommend using SkeletonAnimation and not SkeletonAnimator.
If you are very experienced with Unity and Mecanim and want to use that, you can use SkeletonAnimator to work in that system.
I hope that was straightforward.