Steps:
1. Open attached project
2. Create a Variant out of Base prefab in Assets folder
3. Open Variant in Isolation <-- important step
4. Assign Variant asset to the Reference script (exists on Base prefab asset)
==>
Error: "AssetImporter is referencing an asset from the previous import. This should not happen."

Sometimes Editor crashes with the attached call stack

Also repro in Version 2018.4.8f1 (9bc9d983d803)