I Have Following Error When I M Runnning Following Code

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C#3.5

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 Leptonlibrary;
using System.Collections;

namespace AddressExtractor
{
    public partial class Form2 : Form
    {
        public Form2()
        {
            InitializeComponent();
            LeptonDataLayer.MySqlConnectionString = "server=192.168.1.224; user id=root; password=lepton; database=leptondata; pooling=false;default command timeout=3600;";
        }

        private void button2_Click(object sender, EventArgs e)
        {
            this.Close();
        }

private void button1_Click(object sender, EventArgs e)
       {

if (txtUserName.Text == "")
           {
             MessageBox.Show("Please Enter UserName");
             txtUserName.Focus();
             return;
           }
           if (txtPassword.Text == "")
           {
               MessageBox.Show("Please Enter Password");
               txtPassword.Focus();
               return;
           }
           if (ddlCityName.Text != "")
           {
               this.Hide();
               Form1 frmmain = new Form1();
               frmmain.UserName = txtUserName.Text;
               frmmain.CityName = ddlCityName.Text;
               string cityname = frmmain.CityName;
               string newcituname = string.Empty;
               newcituname = cityname;
               frmmain.NewCityName = newcituname;

string SqlQuery = "Update " + frmmain.CityName + " set  assignedtouserid='" + frmmain.UserName + "', flag=1 where flag=0 and city like '%" + newcituname + "%' ORDER BY LOCALITY DESC limit 100 ";
                LeptonDataLayer.ExecuteNoneQuery(1, SqlQuery);
                SqlQuery = "Update userlogin set  LoginDateTime=NOW() where UserName='" + frmmain.UserName + "'";
                LeptonDataLayer.ExecuteNoneQuery(1, SqlQuery);
                frmmain.ShowDialog();

            }
            else
            { MessageBox.Show("Please Select CityName"); }

        }

        private void txtPassword_Leave(object sender, EventArgs e)
        {
            string SqlQuery = "Select CityName from userlogin where UserName='" + txtUserName.Text + "' and Password='" + txtPassword.Text + "'";
            DataTable dtddl = LeptonDataLayer.ExecuteDataTable(1, SqlQuery);
            if (dtddl.Rows.Count > 0)
            {
                for (int i = 0; i < dtddl.Rows.Count; i++)
                    ddlCityName.Items.Add(dtddl.Rows[i]["CityName"].ToString());
            }

            else
                MessageBox.Show("Incorrect UserName And Password");
        }



    }
}

when i m running the following code i have following error

"Object reference not set to an instance of an object."

in my mysql leptondata database i have a table userlogin which have following field UserName,Password and CityName.


what is problem in above code pls tell me