How to reproduce:
1. Open the attached Unity project
2. Open "SampleScene"
3. Select Sphere's prefab in the scene and go to its edit mode
4. Press "Add Component"
5. Create and add a new script

Expected result: a newly created script is added to the prefab
Actual result: an error message appears; the script is not added to the prefab (can be seen only after closing the prefab edit mode)

Reproducible with - 2018.3.0a6, 2018.3.0a9
Could not test with - 2017.1.5f1, 2017.2.3p3, 2017.4.9f1, 2018.1.9f1, 2018.2.5f1, 2018.3.0a5 (does not have a prefab edit mode)

Note:

Error message:
"You are trying to replace or create a Prefab from the instance 'Sphere' that contains the script 'test', which does not derive from MonoBehaviour. This is not allowed.
Please change the script to derive from MonoBehaviour or remove it from the GameObject.
UnityEditor.EditorApplication:Internal_CallUpdateFunctions()"