I ran into the exact same problem last week using Kentico 9.0.45:
Exception type: System.NullReferenceException
at CMS.DataEngine.SimpleDataClass.CopyOriginalDataTo(IDataClass target)
The master page for this website became unusable after the error and I had to use an earlier clone of the master page to rebuild it. I still have the corrupted master page, but can't find anything wrong (in the database). Any suggestions?