Reproduction steps:
1. Open the attached “IN-98463.zip” project
2. Open the “SampleScene”
3. Select “Main Camera” GameObject from the Hierarchy
4. In the Inspector, in the “Cinemachine Brain” Component find the Custom Blends property
5. Click on the circle next to it and attach the “Main Camera Custom Blends” asset
6. Double-click on the Inspector tab to expand it
7. Observe the Editor and the Console

Expected results: No errors and the Inspector window is maximised
Actual results: “NullReferenceException: Object reference not set to an instance of an object” and the Inspector window is not properly maximised

Reproducible with: 3.0.0-pre.5 (2022.3.61f1, 6000.0.46f1, 6000.1.0b14, 6000.2.0a8), 3.1.3 (2022.3.61f1, 6000.0.46f1, 6000.1.0b14, 6000.2.0a8)
Not reproducible with: 3.0.0-pre.4 (2022.3.61f1, 6000.0.46f1, 6000.1.0b14, 6000.2.0a8)

Reproduced on: Windows 11
Not reproducible on: No other environment tested

Notes:
- After the issue is reproduced, the Custom Blends menu on the Main Camera is blank, and no Custom Blends can be added; {{"InvalidOperationException: Unable to add or remove items because the source is not defined. "}} is thrown
- If the issue does not reproduce after doing the reproduction steps, try to add additional Custom Blends by expanding the Custom Blends property and clicking the + icon, and then retry the 6th step or redo the 5th step by removing and re-adding the asset reference
- The reproduction can be inconsistent, but once it reproduces, everything starts breaking down
- Sometimes, after maximising the Inspector and then unmaximising it, two or more Custom Blends menus would appear in the Inspector