Yeah, you for sure can't use NullableDataReference<IImageFile> as a property-type, this is only supported for properties on functions. For datatypes only the basic types are allowed, that is string, guid, int, decimal etc. (and their nullable counterparts). This means that if you want to store a reference to a mediafile, you need the type of string and then do the media-file lookup or joins later in code yourself.
You said you deleted the id from DynamicXmlProvider.config after having the errors. You should also delete the file from DataMetaData which belonged to your type, since the definition is stored in here and it can be next to impossible sometimes to get C1 to change this file when it first there and the only way out is to delete it.
You said you deleted the id from DynamicXmlProvider.config after having the errors. You should also delete the file from DataMetaData which belonged to your type, since the definition is stored in here and it can be next to impossible sometimes to get C1 to change this file when it first there and the only way out is to delete it.