Introduced on 0a693dba0b96d124edf643ae7e6443d230849569 by https://ono.unity3d.com/unity/unity/pull-request/58414/_/asset-pipeline/mono-importer-remove-compilation-pipeline-dependency

How to reproduce:
1. Create a new Unity project
2. Import SabreCSG Package: https://github.com/sabresaurus/SabreCSG
3. Create CSG Model in a scene
4. Create Assembly Definition inside SabreCSG-masteder folder
5. Select CSGModel in the scene

Result: Errors pop up every time CSGModel or Cube Brush is selected:
Instance of CSGModelInspector couldn't be created. The script class needs to derive from ScriptableObject and be placed in the Assets/Editor folder.
Instance of CSGBuildContextInspector couldn't be created. The script class needs to derive from ScriptableObject and be placed in the Assets/Editor folder.

Note: reimporting project fixes this problem.

Regression introduced in: 2018.2.0a1
Reproduced with: 2018.2.21f1, 2018.3.14f1, 2018.4.6f1, 2019.1.14f1, 2019.2.1f1, 2019.3.0a11.
Not reproduced with: 2017.4.31f1, 2018.1.9f2.

Note: in some versions, you need to select CSGModel or Cube Brush multiple times to receive errors.