How to reproduce:
1. Open the user's attached project and SampleScene scene
2. In the Hierarchy window, select GameObject
3. In the Inspector window, in the Thing dropdown select D
4. In the Project window, open the NewBehaviourScript.cs and comment out the line #17 (definition of class D)
5. Observe the error in the Console window and the value of the Thing dropdown (value is null)
6. Reimport any of the scripts and observe the Console window (the error appears again even though the value is null now)
7. In the Thing dropdown select A and reimport any script again
8. Observe the Console window

Expected result: the error "Unknown managed type referenced: [Assembly-CSharp] .D" appears and the Thing value resets to null
Actual result: there are no errors

Reproducible with: 2019.4.28f1, 2020.3.13f1, 2021.1.13f1, 2021.2.0a18
Not reproducible with: 2021.2.0a19, 2021.2.0b1
Could not test with: 2018.4.36f1 (SerializeReference errors)

Note: If class D is uncommented in the script, the Thing field value is set to D again