Editor crashes on packing master atlas after its settings are changed and when it's used by a packed variant atlas

Steps to Repro:
1. Open attached or create a new project
2. Right-click > create
3. Sprite > square
4. Right-click > sprite atlas
5. Repeat step 4
6. Set one of the atlases as the variant of other
7. Add sprite to master atlas click pack preview
8. Click pack preview for variant sprite
9. Change RGB setting in the master atlas and click pack preview

Actual Result:
1. Observe the crash

Note:
1. The crash occurs after changing any setting in the master atlas

Environment:
Windows and Mac

Occurring with:
2019.1.0a5, 2018.3.0b6, 2018.3.0a9

Working with:
2018.3.0a8, 2018.3.0a1, 2018.2.7f1