Problem statement

I have a spine project imported in Unity and rendered in a prefab.
That project was recently updated and reimported in Unity.
No error from the importer:

skin_lot_3_ville_Atlas :: Imported with 5 material
Changes to 'Assets/Content/Spine/NPC/City/Lot3/skin_lot_3_ville.json' or atlas detected. Clearing SkeletonDataAsset: Assets/Content/Spine/NPC/City/Lot3/skin_lot_3_ville_SkeletonData.asset

After that, when the prefab assign that skeletonData into the SkeletonAnimation, it's triggering a flood of errors in the console when I run the game.

ArgumentNullException: Value cannot be null.
Parameter name: source
UnityEngine.Material..ctor (UnityEngine.Material source) (at <f7237cf7abef49bfbb552d7eb076e422>:0)
Spine.Unity.SkeletonRenderer.InitSpriteMaskMaterialsForMaskType (UnityEngine.Rendering.CompareFunction maskFunction, UnityEngine.Material[]& materialsToFill) (at ./Library/PackageCache/com.esotericsoftware.spine.spine-unity@6adfde3c0f/Runtime/spine-unity/Components/SkeletonRenderer.cs:704)
Spine.Unity.SkeletonRenderer.InitSpriteMaskMaterialsOutsideMask () (at ./Library/PackageCache/com.esotericsoftware.spine.spine-unity@6adfde3c0f/Runtime/spine-unity/Components/SkeletonRenderer.cs:692)
Spine.Unity.SkeletonRenderer.AssignSpriteMaskMaterials () (at ./Library/PackageCache/com.esotericsoftware.spine.spine-unity@6adfde3c0f/Runtime/spine-unity/Components/SkeletonRenderer.cs:680)
Spine.Unity.SkeletonRenderer.LateUpdateMesh () (at ./Library/PackageCache/com.esotericsoftware.spine.spine-unity@6adfde3c0f/Runtime/spine-unity/Components/SkeletonRenderer.cs:566)
Spine.Unity.SkeletonRenderer.LateUpdate () (at ./Library/PackageCache/com.esotericsoftware.spine.spine-unity@6adfde3c0f/Runtime/spine-unity/Components/SkeletonRenderer.cs:459)
Spine.Unity.SkeletonAnimation.LateUpdate () (at ./Library/PackageCache/com.esotericsoftware.spine.spine-unity@6adfde3c0f/Runtime/spine-unity/Components/SkeletonAnimation.cs:283)
Spine.Unity.SkeletonRenderer.Initialize (System.Boolean overwrite, System.Boolean quiet) (at ./Library/PackageCache/com.esotericsoftware.spine.spine-unity@6adfde3c0f/Runtime/spine-unity/Components/SkeletonRenderer.cs:427)
Spine.Unity.SkeletonAnimation.Initialize (System.Boolean overwrite, System.Boolean quiet) (at ./Library/PackageCache/com.esotericsoftware.spine.spine-unity@6adfde3c0f/Runtime/spine-unity/Components/SkeletonAnimation.cs:188)
Hotel.Gameplay.NpcModel.<Rebuild>g__Resolve|39_0 (Spine.Unity.SkeletonDataAsset skeletonData) (at Assets/Content/Hotel/Gameplay/NpcModel.cs:230)
Hotel.Utility.AssetUtil+<>c__DisplayClass1_0`1[T].<LoadAsset>b__0 (UnityEngine.ResourceManagement.AsyncOperations.AsyncOperationHandle`1[TObject] handle) (at Assets/Content/Hotel/Utility/AssetUtil.cs:31)
DelegateList`1[T].Invoke (T res) (at ./Library/PackageCache/com.unity.addressables@1.21.20/Runtime/ResourceManager/Util/DelegateList.cs:75)
UnityEngine.Debug:LogException(Exception)
DelegateList`1:Invoke(AsyncOperationHandle`1) (at ./Library/PackageCache/com.unity.addressables@1.21.20/Runtime/ResourceManager/Util/DelegateList.cs:79)
UnityEngine.ResourceManagement.Util.DelayedActionManager:LateUpdate() (at ./Library/PackageCache/com.unity.addressables@1.21.20/Runtime/ResourceManager/Util/DelayedActionManager.cs:162)

After digging a bit and comparing to other skeleton it seems to be caused by an empty material in the material list of the SkeletonAnimation

Additional Info : There's a Skeleton Render Separator added to the SkeletonAnimation prefab for the characters. In the prefab, the SkeletonAnimation has mask interaction outside mask set.

The previous version of the spine project did not trigger any error. The difference between the previous one and the one causing the errors is a change from spine 4.1.23 to 4.1.24 and the project is more complex than previous one.

Prefab component: SpineAnimation, MeshRenderer, MeshFilter, Skeleton Render Separator, Custom Script, Box Collider

The code I used to swap the SkeletonData:

skeleton.skeletonDataAsset = skeletonData;
skeleton.initialSkinName = SkinName;
skeleton.Initialize(true, true);

Note that this code was working OK with the previous version of the project.

As I could not solve the issue, I removed all Spine related scripts from the prefab and added them using code. The code above became:

skeleton = SkeletonAnimation.AddToGameObject(gameObject, skeletonData);
GetComponent<MeshRenderer>().sortingOrder = 30;
var slot1 = skeleton.Skeleton.FindSlot("Bras_OL");
var slot2 = skeleton.Skeleton.FindSlot("Jambe_UL");
skeleton.separatorSlots.Clear();
skeleton.separatorSlots.Add(slot1);
skeleton.separatorSlots.Add(slot2);
skeleton.Skeleton.SetSkin(SkinName);
                
_separator = SkeletonRenderSeparator.AddToSkeletonRenderer(skeleton);
_separator.enabled = false;

Using that method, the materials in the mesh renderer don't have an empty value and everything is working correctly but it's less convenient and I can't tweak the prefab using the inspector anymore.

I don't know if the problem came from a feature we used in the spine project or from the upgrade from 4.1.23 to 4.1.24.

Any help would be appreciated.

Runtime information

tried with

Spine 4.1.24
spine-unity Runtime 4.1.21
spine-csharp Runtime 4.1.0
Spine Universal RP Shaders 4.1.21

and the latest to date:

spine-unity Runtime 4.1.27
spine-csharp Runtime 4.1.2
Spine Universal RP Shaders 4.1.40

Spinebot The bot seems to imply that it's a bug in the runtime but maybe I'm missing something and what I do to swap the SkeletonData in the SkeletonAnimation is illegal.

Harald Thank you for your answer.

I can already bring some answers.
The updated project has the same amount of materials than the previous version. There was never a _6 in the project history.

When trying to fix the issue, I hit the reload button several time in play mode to see if it made a difference. It did not have any effect.
I also tried removing materials from the inspector, they are removed but it does not change the flood of error in the console. As soon as I hit reload after removing the materials, they are added back with the None in the first slot.

Looking at the inspector for the skeleton and atlas I did not see any warnings or anything.
Here are the folder before and after the project update.

Except the .private file that I deleted because it was left over by Plastic they are similar.

And here are the screenshots of the atlas before and after the update. (Sorry they are quite long)

I don't see anything wrong here.

Harald Sorry I wasn't aware the picture would be resized.

Here are the screenshots.

They look identical before and after the update.

The reason I'm testing in playmode is because the prefab is a generic NPC prefab and its role is to load the correct SkeletonDataAsset in the SkeletonAnimation it contains to display the right skin depending on the NPC data using the code from my first post.
I continued testing outside of playmode and was able to pinpoint exactly when the null material is added to the list. It has to do with the Separator Slot Name.
If I add Jambe_UL in the Separator Slot Names and hit Reload, the null material is added to the list. As soon as I remove it, the null material disappears, even without hitting Reload (and hitting Reload does not add it back at this point)

This works:

This does not:

For a reason I can't explain yet, it does not happen with other slot names. I selected the ones right above and below in the list (Bras_UL or Jambe_OL) and it does not make the null material appear in the list.

Do you have any idea why a specific slot name in the Separator Slot Names would cause that?
I don't think I have something special with that slot in the project

EDIT: the tests above have been done on a "fresh" SkeletonAnimation dragged in the scene from the SkeletonDataAsset, not on my prefab.

I was able to reproduce the issue in a brand new project containing only the spine asset.
If you are interested, tell me how to send you that.

Misaki Hi, the mail has been sent with a Dropbox link to a zip archive.
Thanks for your help.

Harald Hello
Yes it does solve the issue, I don't have errors in the console anymore even when the material 0 in the list is null.

Thank you for your help and for the fix release.