How to reproduce:
1. Create a new Unity project
2. In the Hierarchy, create a Cube GameObject
3. Open the Animation window (Window -> Animation -> Animation)
4. Create a new animation
5. Open the Animator window (Window -> Animation -> Animator)
6. Create Sub-State Machine
7. In the Inspector window, click "Add Behaviour" -> New Script -> Create and Add
8. Observe the results

Expected results: The script is added and there are no errors in the Console
Actual results: The script is not added and there are errors in the Console

Reproducible with: 6000.3.0a6, 6000.3.0b1
Not reproducible with: 6000.0.56f1, 6000.1.16f1, 6000.2.1f1, 6000.3.0a5

Reproducible on: macOS Sequoia 15.5 (M1)
Not reproducible on: No other environment tested

Notes:
- Multiple errors are spawned at once (7-10)
- Errors are preventing adding any behaviour

First line of error:
- NullReferenceException: Object reference not set to an instance of an object
UnityEditor.Graphs.AnimationStateMachine.AddStateMachineBehaviourComponentWindow.get_activeElement () (at /Users/bokken/build/output/unity/unity/Editor/Graphs/UnityEditor.Graphs/AnimationStateMachine/AddStateMachineBehaviourComponentWindow.cs:320)