“Save Changes Failed - Index was out of range” error popup appears when discarding changes in the Sprite Library Editor

Steps to reproduce:

1. Create a new Unity project using the Universal 2D template
2. Open the Sprite Library Editor window (Window > 2D > Sprite Library Editor window)
3. Dock the Sprite Library Editor window
4. Click the "Create new Sprite Library Asset"
5. Click the "+" button 5 times to create 5 Categories
6. Click the "+" button 5 times to create 5 Labels
7. Right click on the Sprite Library Editor window and select "Close Tab"
8. Click the "Discard" button
9. Observe the Editor

Actual results: “Save Changes Failed - Index was out of range” error popup appears

Expected results: No error popup appears about saving changes since the "Discard" button was pressed

Reproducible with versions: 6000.0.67f1, 6000.3.9f1, 6000.4.0b9, 6000.5.0a7

Tested on (OS): MacOS SIlicon Tahoe 26.2, Ubuntu 24.04