How to reproduce:
1. Create new project
2. Open a color picker (e.g. Window → Rendering → Lighting → Environment → Realtime Shadow Color)
3. Create a new color library in Project (Swatches → Three Dots → Create New Library)
4. Select the created color library (Project\Editor)
5. In the Inspector press Edit
6. Add a new swatch
7. Observe the console

Expected result: No error is thrown
Actual result: “NullReferenceException: Object reference not set to an instance of an object” is thrown every refresh frame

Reproducible with: 6000.2.0a7, 6000.2.13f1, 6000.3.0f1, 6000.4.0a5, 6000.5.0a1
Not reproducible with: 6000.2.0a6, 6000.0.62f1

Reproducible on: Windows 11 Pro (24H2)
Not reproducible on: No other environments tested