Simple Code:

    public static void main(){
        string f = "abc def";
        try{
            foreach(var s in f.split(" ")){
                message("%s", s); 
                unowned Thread t = Thread.create(()=>{
                        message("%s", s); 
                        return null;
                        }, true);
                t.join();
            }   
        }catch(Error e){ 
        }   
    }
=======================================================
In both message, it showed 'null'.

valac --version
Vala 0.10.1