So ever since updating to Unity 2017 I've had this problem. At first I waited for updates to come out to see if that would remedy the issue. However, downloading Spine.Unity runtime 3.6 (and using both unity 2017.1 and spine 3.6 like the runtimes state) did not seem to solve the issue. To further test this I created a new project containing nothing outside of the spine.unity runtimes and a freshly exported spine animation.
attached is an image of the errors in question. Specifically when I highlight the skeletonData (or perhaps because it's being displayed in the inspector) the NullReferenceException starts popping up at an alarming rate (8k was just when I took the photo, since typing this its risen to 70k). Sometimes clicking onto something else to change the inspector remedies this, other times it doesn't.
I've been unable to find a solution to this and so I figured I'd post either to ask for a solution, or to make you aware of this problem as it must not be a universal issue.
Thanks in advance and keep up the great work guys!!