How to reproduce:
1. Open the "CreatePrefab.zip" project
2. Delete "Assets/MyPrefab" asset
3. MyCustomMenu > Create Prefab, notice that no errors are thrown
4. Delete "Assets/MyPrefab" asset
5. Reimport "Assets/MyCustomAsset", notice the "prefab == null" error

Actual result: PrefabUtility.CreatePrefab references to null if called via ScriptedImporter.OnImportAsset
Expected result: PrefabUtility.CreatePrefab should refer to the recently created prefab

Reproducible with: 2018.3.0a6, 2018.2.0x-ImprovedPrefabs.
Not reproducible with: 2018.2.1f1, 2018.1.9f1, 2017.4.8f1.

Notes:
MyCustomImporter.cs calls "CreatePrefab()" (line 20) function on reimport or when "Create Prefab" menu option is pressed, which creates prefab.