I had the same issue with EMS 9.0.38:
Exception type: System.NullReferenceException
at CMS.DataEngine.SimpleDataClass.CopyOriginalDataTo(IDataClass target)
The Event URL referrer is /CMSModules/PortalEngine/UI/WebParts/webpartproperties_properties.aspx
Kentico support suggested clearing the cache. This worked and allowed successful saves. I am not clear on why clearing the cache worked but at least it is something to try.