To repro:

1. Open project to scene 'Tiles'
2. Select 'spritesheet' in assets, change its texture type to 'sprite'
3. Right click on spritesheet, select reimport.
4. Select Window/Sprite Editor with spritesheet still selected
5. Select 'Slice' in upper left corner, choose Type: Grid, Pivot: Top Left.
6. Close Slice window and hit Apply in the top right of Sprite Editor window. (this generates error)
7. Return to the slice window, hit the 'slice' button again. (this re-generates error)