[godot] Crash on re-importing modified skeletons sharing the same atlas.

[metaphore]

Hey there!

I noticed that the Godot editor started to constantly crash when I update and re-export one of my Spine projects.

I narrowed it down to these conditions:

1. There should be multiple skeletons in the same Spine project.
2. They must share one atlas (export with atlas packed as "single atlas").
3. Either of the skeletons should be present (as SpineSprite) on any of the opened scenes in Godot Editor.

I use Spine as Godot engine module (downloaded the pre-build Godot engine from Spine website).
But there were no errors in the console and nothing in logs. So I testing it with custom build to get more context.

• Linux (Pop!_OS 22.04 x86_64)
• Spine Editor 4.2.39
• Spine Runtime #c45adf9dbb2035b68cbb5190bf49d039a3825805 (used to build the Godot editor)
• Godot 4.3-stable (#77dcf97d82cbfe4e4615475fa52ca03da645dbd8) with C# support.

Here's a minimal project to reproduce the issue.
It has a Spine file (asset_sources/test_skeleton.spine) that contains two skeletons (first and second). Each has one bone, one slot and one region attachment (different images).
Skeletons are exported to skeleton/ dir. Each has a related SpineSkeletonDataResource asset pointing to the same atlas - test_skeleton.atlas (SpineAtlasResource).
The main scene (main_screen.tscn) is set up with only one SpineSprite node pointing to the first skeleton.

Steps to reproduce (they are also displayed in the video):

1. Open test project in Godot and open the main scene.
2. Open the asset_sources/test_skeleton.spine Spine project.
3. Add a new image to the first skeleton. The image must be previously unused in any of the skeletons to trigger atlas re-packing. So use skateboard0.
4. Export the project to ./skeleton/ with Single atlas option selected.
5. Return to Godot editor and you will see a "reading attachment error" message in output (or you won't see the error, but the currently active skeleton on the scene will be broken and disappear).
6. Come back to Spine editor and remove previously added image (the slot).
7. Export Spine project with the previous settings.
8. This time Godot editor will crash on import attempt.

screen_cast-2025-02-08_16.36.48-cut.mp4

From my research it doesn't matter weather to export skeletons as binary or json, nor the type of the new attachment is important. Looks like it all comes down to the atlas itself and something isn't right when it's changing for a skeleton on the active Godot scene.

@badlogic please let me know if there's anything else I can help with.