g_object_unref: assertion `G_IS_OBJECT (object)' failed

using Gee;


public abstract class cAbstractObject<G> : Object, Iterable<G> {
    protected  string _name;
    protected  ArrayList<G> _objects = new ArrayList<G>();

    public string name { get { return this._name; } set { this._name = value; } }
    public ArrayList<G> objects { get { return  _objects; } }


    public int size { get { return _objects.size; } }

    public Type element_type {
        get { return typeof (G); }
    }

    public Iterator<G> iterator () {
        return _objects.iterator();
    }

    public virtual void add(G o) {
        _objects.add(o);
    }

    public new G? get (int index) {
        assert (index >= 0); assert (index < _objects.size);
        return  _objects[index];
    }

}

public class cObject : cAbstractObject<cObject>  {
    public  cObject(string name) {
        this._name = name;
    }

    public new void add(cObject o) {
        stdout.printf ("adding %s\n", o.name);
        _objects.add(o);
    }

}

void main() {
    cObject o1 = new cObject("test1");

    o1.add(new cObject("test2"));

    cObject o2 = o1.get(0);
    stdout.printf ("++--> %s\n", o2.name);

}