Highly pixelated sprite- sheet appears in sprite editor when mip maps is checked.

Steps To reproduce:

1. Launch the editor.
2. Import a sprite sheet in the project window.
3. Make sure the 'texture Type' is sprite 2D.
4. Check 'Mip Maps' and hit apply.
5. Open the sprite in the sprite editor.
6. Observe that highly pixelated sprite sheet appears in the sprite editor.

Reproducible with : 2018.1.0b6, 2018.1.0b5, 2018.1.0b4
Works fine with : 2017.3.0f3, 2017.3.0f2, 2018.1.0a7, 2018.1.0a5

Environment: Windows and Mac

License type: Pro