WarningMessage Property for APRegister not found

Why is the WarningMessage property for APRegister not found in APDocumentRelease

I'm working on APDocumentRelease on AppData.CodeRepository.PX.Objects.AP folder. On line 255, the code is giving error since WarningMessage property for doc is not found. I override the ReleaseDoc Method and tried to retain the code snippet but it gives an error if I included that line.