To reproduce:
1. Open any project
2. Import the script attached "CreatePrefab.cs" (or make your own: call PrefabUtility.SaveAsPrefabAsset() from method OnValidate())
3. Assign the script to a game object and pick a game object for the script's public field (ex.: freshly created 3d object)

Result: editor freezes on trying to validate script after changes have been made

Reproduced in: 2018.3.7f1, 2019.1.0b5, 2019.2.0a6

Note1: OnValidate() can also be checked by commenting out PrefabUtility.SaveAsPrefabAsset(), adding GameObject to script's public field and decommenting. OnValidate
will once again freeze the editor.
Note2: PrefabUtility not available on LTS