C# ASPxComboBox6模糊查询解决方案

C# ASPxComboBox6模糊查询
 protected void ASPxComboBox6_TextChanged(object sender, EventArgs e)
    {
          string NumDrop = this.ASPxComboBox6.Text.Trim();
           
          string carnum_query = "SELECT   car_num, car_type, driver, tel, id FROM dbo.diaodu_car_base WHERE car_num LIKE '%" + NumDrop + "%'";

          Response.Write(carnum_query);

          string strConnnection = System.Configuration.ConfigurationManager.ConnectionStrings["emailConnectionString"].ConnectionString;
          SqlConnection con = new SqlConnection(strConnnection);

          //创建OleDbDataAdapter对象，按照指定的查询语句获取结果
          SqlDataAdapter cmd = new SqlDataAdapter(carnum_query, con);

          //定义DataSet对象，将查询结果填充到这个对象上
          DataSet ds = new DataSet();
          cmd.Fill(ds, "contents");

          //绑定数据源
          this.ASPxComboBox6.DataSourceID = null;
          this.ASPxComboBox6.ID  = "";

          this.ASPxComboBox6.DataSource = ds.Tables["contents"].DefaultView;
          this.ASPxComboBox6.ID  = "id";

          this.ASPxComboBox6.DataBind();
        
                 if (con.State == ConnectionState.Closed)
                 {
                     con.Open();
                 }
              
                 List<string> car_num = new List<string>();
                 foreach (DataRow row in ds.Tables[0].Rows)
                 {
                     foreach (DataColumn column in ds.Tables[0].Columns)
                     {
                         car_num.Add(row[column].ToString());
                     }
                 }
                 int index=0;
                 for (int i = 0; i < car_num.Count; i++)
                 {
                     if (car_num[i] == NumDrop)
                     {
                         index = i;
                       
                     }
                 }
                 this.ASPxComboBox6.DataSource = ds  ;
                 this.ASPxComboBox6.SelectedIndex = index ;
}
运行提示错误：The DataSourceID property of 'ASPxComboBox6' must refer to an existing control.  There is no control with the specified ID: '(无)'.    哪里的问题？