I had a domain object
Where Rso itself is another domain object.
Both projectTitle and rso could not be blank or null.
when I tried
project.projectTitle.blank message in messages.properties, it worked
project.rso.null was not working.
I was trying project.rso.null because the default message was mapped as default.null.message.
Then someone suggested me to use
project.rso.nullable and it worked.