[NullReferenceException: 未将对象引用设置到对象的实例。]

Label _UId = new Label();
_UId.Text = Session["UId"].ToString();
Label _dt = new Label();
_dt.Text = DateTime.Now.ToLongTimeString();
Label _words = new Label();
_words.Text = TextBox1.Text;
_words.Style.Add("color", ddl_color.SelectedValue);
Literal lbr = new Literal();
lbr.Text = "<br/>";
Application.Lock();
((Panel)Application["chat"]).Controls.AddAt(0,lbr);
((Panel)Application["chat"]).Controls.AddAt(0,_words);
((Panel)Application["chat"]).Controls.AddAt(0,_dt);
((Panel)Application["chat"]).Controls.AddAt(0,_UId);
Application.UnLock();
Label _UId = new Label();
_UId.Text = Session["UId"].ToString();
Label _dt = new Label();
_dt.Text = DateTime.Now.ToLongTimeString();
Label _words = new Label();
_words.Text = TextBox1.Text;
_words.Style.Add("color", ddl_color.SelectedValue);
Literal lbr = new Literal();
lbr.Text = "<br/>";
Application.Lock();
((Panel)Application["chat"]).Controls.AddAt(0,lbr);
((Panel)Application["chat"]).Controls.AddAt(0,_words);
((Panel)Application["chat"]).Controls.AddAt(0,_dt);
((Panel)Application["chat"]).Controls.AddAt(0,_UId);
Application.UnLock();
上面的代码最后输出的语句有什么问题么，怎么改？
大神求教