กดปุ่ม Add Rowใหม่ในGridView แล้วข้อมูลที่กรอกไว้มันหาย ต้องแก้ยังไงครับ

 Dim dt As New DataTable
 Dim dr As DataRow

    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load

        If (Me.Session("x") Is Nothing) Then
          
            dt = New DataTable

            dt.Columns.Add("A") 
            dt.Columns.Add("B")  
            dt.Columns.Add("C")

            dr = dt.NewRow
            dr("A") = "a"
            dr("B") = "b"
            dr("C") = "c"
      
            Me.Session("x") = dt
  
        Else
            dt = Me.Session("x")
        End If     

    End Sub


    Protected Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button1.Click
        
            dr = dt.NewRow()
            dr("A") = ""
            dr("B") = ""
            dr("C") = ""
        dt.Rows.Add(dr)

            GridView1.DataSource = dt
            GridView1.DataBind()

    End Sub