Steps to reproduce:

1. Create a new 2D URP template project
2. Import the attached .aseprite asset
3. Select the Asset in the Project Browser window
4. In the Inspector window change the import mode to Tile Set
5. Press Apply to save the changes
6. Change the import mode back to Animated Sprite and Apply the changes

Actual results: File import fails and an 'Index was outside the bounds of the array' error is thrown

Expected results: File import succeeds without errors or warnings

Reproducible with versions: 6000.1.0b12, 6000.2.0a7

Can’t test with versions: 2022.3.60f1, 6000.0.44f1 (Tile set option is not available)

Tested on (OS): macOS 15.3.2, Ubuntu 24

Notes:

• After the issue occurs the asset breaks inside the Editor. Re-import or importer reset is needed to fix this

Full error message:
{quote}Failed to import file Assets/Sprite-0001.aseprite. Error: Index was outside the bounds of the array. 
  at UnityEditor.U2D.Aseprite.AsepriteImporter.OnImportAsset (UnityEditor.AssetImporters.AssetImportContext ctx) [0x001b7] in ./Library/PackageCache/com.unity.2d.aseprite@2f6210f13ac2/Editor/AsepriteImporter.cs:240 
UnityEngine.GUIUtility:ProcessEvent (int,intptr,bool&) (at /Users/bokken/build/output/unity/unity/Modules/IMGUI/GUIUtility.cs:219)
{quote}