I used the example project.
I also used the example code.

OptimizeSkin After doing it several times, at some point, when I OptimizeSkin, the skin breaks.

Or it gets blurry.

The spine file has many skins and has more than 13 materials. It's version 4.2.
ㅠ0ㅠ I can't solve the problem..ㅠㅠㅠ

MissingReferenceException: The object of type 'UnityEngine.Material' has been destroyed but you are still trying to access it.
Your script should either check if it is null or you should not destroy the object.
UnityEngine.Object+MarshalledUnityObject.TryThrowEditorNullExceptionObject (UnityEngine.Object unityObj, System.String parameterName) (at <2431722e3e3e41d78b6718bb39ab9111>:0)
UnityEngine.Bindings.ThrowHelper.ThrowNullReferenceException (System.Object obj) (at <2431722e3e3e41d78b6718bb39ab9111>:0)
UnityEngine.Material.GetFirstPropertyNameIdByAttribute (UnityEngine.Rendering.ShaderPropertyFlags attributeFlag) (at <2431722e3e3e41d78b6718bb39ab9111>:0)
UnityEngine.Material.get_mainTexture () (at <2431722e3e3e41d78b6718bb39ab9111>:0)
Spine.Unity.AttachmentTools.AtlasUtilities.GetMainTexture (Spine.AtlasRegion region) (at Assets/Spine/Runtime/spine-unity/Utility/AtlasUtilities.cs:819)
Spine.Unity.AttachmentTools.AtlasUtilities.ToTexture (Spine.AtlasRegion ar, UnityEngine.TextureFormat textureFormat, System.Boolean mipmaps, System.Int32 texturePropertyId, System.Boolean linear, System.Boolean applyPMA) (at Assets/Spine/Runtime/spine-unity/Utility/AtlasUtilities.cs:611)
Spine.Unity.AttachmentTools.AtlasUtilities.AddRegionTexturesToPack (System.Int32 numTextureParamsToRepack, Spine.AtlasRegion region, UnityEngine.TextureFormat textureFormat, System.Boolean mipmaps, UnityEngine.TextureFormat[] additionalTextureFormats, System.Int32[] additionalTexturePropertyIDsToCopy, System.Boolean[] additionalTextureIsLinear) (at Assets/Spine/Runtime/spine-unity/Utility/AtlasUtilities.cs:477)
Spine.Unity.AttachmentTools.AtlasUtilities.GetRepackedAttachments (System.Collections.Generic.List1[T] sourceAttachments, System.Collections.Generic.List1[T] outputAttachments, UnityEngine.Shader shader, UnityEngine.Material& outputMaterial, UnityEngine.Texture2D& outputTexture, System.Int32 maxAtlasSize, System.Int32 padding, UnityEngine.TextureFormat textureFormat, System.Boolean mipmaps, System.String newAssetName, UnityEngine.Material materialPropertySource, System.Boolean clearCache, System.Boolean useOriginalNonrenderables, System.Int32[] additionalTexturePropertyIDsToCopy, UnityEngine.Texture2D[] additionalOutputTextures, UnityEngine.TextureFormat[] additionalTextureFormats, System.Boolean[] additionalTextureIsLinear) (at Assets/Spine/Runtime/spine-unity/Utility/AtlasUtilities.cs:377)
Spine.Unity.AttachmentTools.AtlasUtilities.GetRepackedAttachments (System.Collections.Generic.List1[T] sourceAttachments, System.Collections.Generic.List1[T] outputAttachments, UnityEngine.Material materialPropertySource, UnityEngine.Material& outputMaterial, UnityEngine.Texture2D& outputTexture, System.Int32 maxAtlasSize, System.Int32 padding, UnityEngine.TextureFormat textureFormat, System.Boolean mipmaps, System.String newAssetName, System.Boolean clearCache, System.Boolean useOriginalNonrenderables, System.Int32[] additionalTexturePropertyIDsToCopy, UnityEngine.Texture2D[] additionalOutputTextures, UnityEngine.TextureFormat[] additionalTextureFormats, System.Boolean[] additionalTextureIsLinear) (at Assets/Spine/Runtime/spine-unity/Utility/AtlasUtilities.cs:279)
Spine.Unity.AttachmentTools.AtlasUtilities.GetRepackedSkin (Spine.Skin o, System.String newName, UnityEngine.Shader shader, UnityEngine.Material& outputMaterial, UnityEngine.Texture2D& outputTexture, System.Int32 maxAtlasSize, System.Int32 padding, UnityEngine.TextureFormat textureFormat, System.Boolean mipmaps, UnityEngine.Material materialPropertySource, System.Boolean clearCache, System.Boolean useOriginalNonrenderables, System.Int32[] additionalTexturePropertyIDsToCopy, UnityEngine.Texture2D[] additionalOutputTextures, UnityEngine.TextureFormat[] additionalTextureFormats, System.Boolean[] additionalTextureIsLinear) (at Assets/Spine/Runtime/spine-unity/Utility/AtlasUtilities.cs:550)
Spine.Unity.AttachmentTools.AtlasUtilities.GetRepackedSkin (Spine.Skin o, System.String newName, UnityEngine.Material materialPropertySource, UnityEngine.Material& outputMaterial, UnityEngine.Texture2D& outputTexture, System.Int32 maxAtlasSize, System.Int32 padding, UnityEngine.TextureFormat textureFormat, System.Boolean mipmaps, System.Boolean useOriginalNonrenderables, System.Boolean clearCache, System.Int32[] additionalTexturePropertyIDsToCopy, UnityEngine.Texture2D[] additionalOutputTextures, UnityEngine.TextureFormat[] additionalTextureFormats, System.Boolean[] additionalTextureIsLinear) (at Assets/Spine/Runtime/spine-unity/Utility/AtlasUtilities.cs:521)
SpineSkinManager.OptimizeSkin () (at Assets/Scripts/Spine/SpineSkinManager.cs:101)
UnityEngine.Events.InvokableCall.Invoke () (at <2431722e3e3e41d78b6718bb39ab9111>:0)
UnityEngine.Events.UnityEvent.Invoke () (at <2431722e3e3e41d78b6718bb39ab9111>:0)
UnityEngine.UI.Button.Press () (at ./Library/PackageCache/com.unity.ugui@22ebcc83720a/Runtime/UGUI/UI/Core/Button.cs:70)
UnityEngine.UI.Button.OnPointerClick (UnityEngine.EventSystems.PointerEventData eventData) (at ./Library/PackageCache/com.unity.ugui@22ebcc83720a/Runtime/UGUI/UI/Core/Button.cs:114)
UnityEngine.EventSystems.ExecuteEvents.Execute (UnityEngine.EventSystems.IPointerClickHandler handler, UnityEngine.EventSystems.BaseEventData eventData) (at ./Library/PackageCache/com.unity.ugui@22ebcc83720a/Runtime/UGUI/EventSystem/ExecuteEvents.cs:57)
UnityEngine.EventSystems.ExecuteEvents.Execute[T] (UnityEngine.GameObject target, UnityEngine.EventSystems.BaseEventData eventData, UnityEngine.EventSystems.ExecuteEvents+EventFunction`1[T1] functor) (at ./Library/PackageCache/com.unity.ugui@22ebcc83720a/Runtime/UGUI/EventSystem/ExecuteEvents.cs:272)

UnityEngine.EventSystems.EventSystem:Update() (at ./Library/PackageCache/com.unity.ugui@22ebcc83720a/Runtime/UGUI/EventSystem/EventSystem.cs:530)