Null Reference Exception occurs on creating a new color preset library.

Steps to reproduce :
1. Open the attached project or create new.
2. Right-click in Project window > Create > Material
3. Open Albedo color window from the inspector.
4. Click Swatches context menu in the bottom-right corner of the color window.
5. Click Create New Library option from the drop-down menu, enter the name and click Create button.

Actual Result:
Null Reference Exception occurs on creating a new color preset library.

Expected Result :
Null Reference Exception should not occur on creating a new color preset library.

Occurring On: 2019.1.0a9

Works fine On: 2019.1.0a8, 2018.3.0b10

Environment: Windows and Mac