[unity] GetRepackedSkin NullReferenceException.

spine-unity/Assets/spine-unity/Modules/AttachmentTools/AttachmentTools.cs

@@ -486,6 +486,7 @@ namespace Spine.Unity.Modules.AttachmentTools {
 		/// Creates and populates a duplicate skin with cloned attachments that are backed by a new packed texture atlas comprised of all the regions from the original skin.</summary>
 		/// <remarks>No Spine.Atlas object is created so there is no way to find AtlasRegions except through the Attachments using them.</remarks>
 		public static Skin GetRepackedSkin (this Skin o, string newName, Shader shader, out Material outputMaterial, out Texture2D outputTexture, int maxAtlasSize = 1024, int padding = 2, TextureFormat textureFormat = SpineTextureFormat, bool mipmaps = UseMipMaps, Material materialPropertySource = null, bool clearCache = false) {
+			if (o == null) throw new System.NullReferenceException("Skin was null");
 			var skinAttachments = o.Attachments;
 			var newSkin = new Skin(newName);