How to reproduce:
1. Open the attached '1319708.zip' project
2. In the Project window double-click on the Test Animator Controller
3. In the Animator window select the 'A' State
4. Observe the Inspector
5. Open Assets > DummyScript.cs
6. Change the base from 'StateMachineBehaviour' to 'MonoBehaviour'
7. Save the Script
8. Switch back to the Unity Editor
9. Select the 'A' State
10. Observe the Inspector

Expected result: The 'Add Behaviour' button is displayed in the Inspector, no visible errors present
Actual result: The 'Add Behaviour' button and the 'DummyScript' component are not displayed in the Inspector, multiple ArgumentNullException errors appear in the Console

Reproducible with: 2018.4.32f1, 2019.4.22f1, 2020.2.7f1, 2020.3.0f1, 2021.1.0b11, 2021.2.0a8

Notes:
- The following error gets thrown in the Console:
ArgumentNullException: Value cannot be null.
Parameter name: componentOrGameObject
- The issue doesn't reproduce when changing the base class to 'ScriptableObject'