Connection issue in C# WinForms application with multiple forms
if (usertype == "UT1") //admin rights
{
    //GET LOGGED USER
    Home_Admin homeAdmin = new Home_Admin();
    homeAdmin.SetUsername(username);

    cString.Close();
    this.Close();

    System.Threading.Thread t = new System.Threading.Thread(new System.Threading.ThreadStart(OpenHomeAdmin));
    t.Start();
}

private void backUpToolStripMenuItem_Click(object sender, EventArgs e)
{
    BackUp BackUpForm = new BackUp();
    BackUpForm.Show();
}

if(conn.State = connectionState.Open)
{
     conn.close();
}

public void BackupDatabase(String destinationPath)
    {
        SqlConnection cString = new SqlConnection();
        cString.ConnectionString = "Data Source=.\SQLEXPRESS;AttachDbFilename=D:\MY_THESIS\WORKING FILES\NNIT-RMS.mdf;Integrated Security=True;Connect Timeout=30;User Instance=True";

        if (cString.State == ConnectionState.Open)
        {
            cString.Close();
        }

        try
        {
            //MY SERVER
            String userName = "NNIT-Admin";
            String password = "password";
            String serverName = @"RITZEL-PCSQLEXPRESS";

            ServerConnection connection = new ServerConnection(serverName, userName, password);
            Server sqlServer = new Server(connection);

            Backup BackupMgr = new Backup();
            BackupMgr.Devices.AddDevice(destinationPath, DeviceType.File);
            BackupMgr.Database = "NNIT DB";
            BackupMgr.Action = BackupActionType.Database;
            BackupMgr.SqlBackup(sqlServer);

            MessageBox.Show("Back up saved!");
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.Message + " " + ex.InnerException);
        }
    }

private void BackUp_Load(object sender, EventArgs e)
{
    string date = DateTime.Now.Day.ToString();
    string year = DateTime.Now.Year.ToString();
    string month = DateTime.Now.Month.ToString();

    Filename_txt.Text = "NNIT-RMSDB_" + month + date + year;
}

SqlConnection cString = new SqlConnection();

cString.ConnectionString = "Data Source=.\SQLEXPRESS;AttachDbFilename=D:\MY_THESIS\WORKING FILES\NNIT-RMS.mdf;Integrated Security=True;Connect Timeout=30;User Instance=True";

if (cString.State == ConnectionState.Open)
{
    cString.Close();
}