Reproduction steps:
1. Open the user’s attached project
2. Click Assets/Opsive/UltimateCharacterController/Demo/Prefabs/Items/FragGrenade/LeftFragGrenade.prefab
3. In the Inspector window, under the Throwable Action Script, under “Trigger Action Module Group” click on the “NULL” element
4. Click “-”

Expected result: NULL element is removed
Actual result: Unity Editor crashes

Reproducible with: 2021.3.0f1
Not reproducible with: 2021.2.19f1, 2021.3.1f1, 2021.3.7f1, 2022.1.12f1, 2022.2.0b3, 2023.1.0a5
Couldn't test with: 2020.3.37f1 (couldn’t downgrade)

Reproducible on: Windows 10 Pro 21H2

First lines of the stack trace:

{noformat}0x00007ff710c01bb3 (Unity) IsTypeTreePPtr
0x00007ff711aed65f (Unity) CollectPropertyValuesFromSerializedObject
0x00007ff711af34dd (Unity) GeneratePropertyDiffInternal
0x00007ff711af3cd5 (Unity) GeneratePropertyDiffInternal
0x00007ff711af2491 (Unity) GeneratePropertyDiff{noformat}