Patched and sumbitting pull request to github right now.
In the mean time in SpineEditorUtilities.cs find:
skin = skeletonDataAsset.GetSkeletonData(true).DefaultSkin;
Add after:
if(skin == null)
skin = skeletonDataAsset.GetSkeletonData(true).Skins[0];
Error occurs when a DefaultSkin isn't serialized in JSON (ie: you only have data in a custom skin, but none in the "No skins selected" state of the spine editor.