private void btnDN_ItemClick(object sender, DevExpress.XtraBars.ItemClickEventArgs e)
{
Form frm = this.KiemTraTonTai(typeof(frmDangNhap));
if (frm != null)
{
frm.Activate();
}
else
{
frmDangNhap dn = new frmDangNhap();
dn.MdiParent = this;
dn.Dock = DockStyle.Fill;
dn.Show();
}
}