I've got the attached stack trace when using the "Settings"-> "Importer" page in the GeoServer Admin UI
This doesn't cause any misbehavior, the UI is correctly reading and writing the importer/importer.properties file
Get the latest Platform Independent Binary
Add the Importer (Core) plugin
Login to the admin interface
Click on the "Settings"-> "Importer"
Java version "1.8.0_231"