autocad.net(C#)自定义的命令只能调用一次，再次调用，出现如下错误：eNullObjectId

autocad.net(C#)自定义的命令只能调用一次，将第一次画的直线删除后，再次调用画直线就不给画了，出现如下错误：eNullObjectId。
刚刚学习编程，比较菜，请高手指点一下。
下面是我写画直线的函数

   public static ObjectId drawLine2pt(Point3d Pt1,Point3d Pt2,Color LineColor,ObjectId LayerId)
        {
            MyDoc doc = MyApp.DocumentManager.MdiActiveDocument;
            MyEditor ed = doc.Editor;
            Mydatabase db = doc.Database;
            ObjectId obid = ObjectId.Null ;
            using(Transaction trans=db.TransactionManager .StartTransaction())
            {
                try
                {
                    BlockTable bt;
                    bt = trans.GetObject(db.BlockTableId, OpenMode.ForRead) as BlockTable;

                    BlockTableRecord btr;
                    btr = trans.GetObject(bt[BlockTableRecord.ModelSpace] , OpenMode.ForWrite) as BlockTableRecord;

                    Line ln = new Line(Pt1, Pt2);
                    ln.Color = LineColor;
                    ln.LayerId = LayerId;
                    obid = btr.AppendEntity(ln);
                    trans.AddNewlyCreatedDBObject(ln, true);
                    trans.Commit();

                }
                catch (System.Exception ex)
                {
                    ed.WriteMessage("发生如下错误：" + ex.Message);
                    trans.Abort ();
                }
            }
            return obid;
        }

btr = trans.GetObject(bt[BlockTableRecord.ModelSpace] , OpenMode.ForWrite) as BlockTableRecord;
调试下这里BlockTableRecord.ModelSpace是不是有问题，没有找到对象