Error:没有可用的错误消息

一个添加用户程序：
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Data.OleDb;
using System.Data.SqlClient;
namespace store
{
    public partial class Adduser : Form
    {
        public Adduser()
        {
            InitializeComponent();
            
        }        OleDbConnection coon = new OleDbConnection("Provider=SQLOLEDB; Data Source=.;Initial Catalog=store;Integrated Security=False");
        private void button1_Click(object sender, EventArgs e)
        {
            if (textname.Text.Trim() == "" || textpassword.Text.Trim() == "" || textnew.Text.Trim() == "")
            {
                MessageBox.Show("请输入用户名和密码!");
            }
            else
            {
                if (textpassword.Text.Trim() != textnew.Text.Trim())
                {
                    MessageBox.Show("两次密码不一致，请重新输入！");
                }
                else
                {
                    try
                    {
                        
                        string sql = "SELECT * FROM user WHERE username='" + textname.Text.Trim() + "'";                        OleDbCommand cmd = new OleDbCommand(sql, coon);
                        coon.Open();
                       //SqlDataReader dr=   cmd.ExecuteReader();
                        if (cmd.ExecuteScalar() == null)
                        {
                            string sql1 = "insert into user(username,password) " +
                                "values ('" + textname.Text.Trim() + "','" + textpassword.Text.Trim() + "')";
                            cmd.CommandText = sql1;
                            cmd.ExecuteNonQuery();
                            MessageBox.Show("添加用户成功！", "提示");
                            coon.Close();
                        }
                        else
                        {
                            MessageBox.Show("用户名已存在，请重新输入！");
                            coon.Close();                        }
                    }
                    catch (Exception ex)
                    {
                        MessageBox.Show(string.Format("Error:{0}", ex.Message), this.Text, MessageBoxButtons.OK);
                    }
                    finally
                    {
                        //coon.Close();
                    }
                }
            }        }
        private void AddUser_Load(object sender, System.EventArgs e)
        {
            DataSet ds = new DataSet();
            OleDbDataAdapter adp = new OleDbDataAdapter("", coon);
            adp.SelectCommand.CommandText = "select username from user";
            adp.Fill(ds);                 }
    }
}
麻烦大家帮我指导一下，谢谢！