Performing an ADPrep caused GPO's to become labeled as "Noncompliant - UnAuthorized modifications" and performing the Undo Check Out produces an error "Value cannot be Null"
On any GPOs that are marked as 'Noncompliant - Unauthorized Modifications' you can run a 'Compliance Check', or a 'Difference Report' to verify that no inappropriate modifications have been done. If these come back clean this indicates the Watcher Service noticed a potential change when the schema update touched the GPOs.
If this is the case you can go into the UI under the version control root and select 'Incorporate Live' to merge the good live version into the version controlled version which will then reset the hash that the Watcher Service used to check for changes. After this they will be marked as compliant properly.