How to reproduce:
1. Create a new project
2. Create an empty GameObject
3. Drag the created GameObject to the Project window to make it a Prefab
4. Select the Prefab in the Project window
5. Lock the Inspector
6. Add a Box Collider Component

Expected result: No errors thrown
Actual result: "The previous instance of UnityEditor.PrefabImporterEditor has not been disposed correctly." errors thrown

Reproducible with: 2019.3.0b6, 2019.3.7f1, 2020.1.0b3, 2020.2.0a4
Not reproducible with: 2017.4.38f1, 2018.4.19f1, 2019.3.0b5

Note: If the bug does not reproduce when adding the Component, delete that Component and add it again (or add other Component), on some version it is reproducible when adding the Component for the second time. On mac it is reproducible from 2020.1.0a20