The script that causes the crash is taken from the asset Pixel Rotation Sprite: https://www.assetstore.unity3d.com/en/#!/content/88118

To reproduce:

1. Open the project, attached by the tester (maskCrash.zip)
2. Open the "Gun_Pointing_Example" scene
3. Select "Arm+Gun" game object in the Hierarchy
4. Add UI Mask component

Expected: Adding a Mask to the game object that has a specific script on it doesn't crash the Editor

Reproduced in 5.6.0f1, 5.6.0f3, 2017.1.0b7
Did not reproduced in 5.5.4f1, 5.6.0b11, 2017.1.0b8, 2017.1.0b10, 2017.2.0a4
Regression introduced in 5.6.0f1

Fixed in 2017.1.0b8