To reproduce:

1. Download attached "1172619_repro (1) 3.zip" project in Unity
2. In Project window select "untitled.fbx" gameObject
3. In Inspector press Materials button, to go to Model Importer Material Editor
4. In Material field press dot button
5. In the opened window try to select "None" Material
6. Observe that in the Material field this Material is shown as "None (Material)"
7. Press again on None selection

Expected results: "None (Material)" Material is still shown in the Material field
Actual results: Now you can see "None (Object)" in in the Material field and you can Apply it

Notes:
- Video for Reference is attached
- This bug might be introduced earlier (2019.3.10f1) but in 2019.3.12f1 if you select "None (Object)" you cannot Apply it and once the Material selection Window is closed, "None (Object)" reverts to "None (Material)"

Reproduced on Unity 2019.3.13f1, 2020.1.0b7 and 2020.2.0a9
Not reproduced on Unity 2019.3.12f1
Regression on Unity 2019.3.13f1