private void btnDangNhap_ItemClick(object sender, DevExpress.XtraBars.ItemClickEventArgs e)
{
Form frm = this.CheckExists(typeof(frmDangNhap));
if (frm != null)
{
frm.Activate();
}
else
{
frmDangNhap f = new frmDangNhap()
{
MdiParent = this, Text = "Đăng nhập"
};
f.Show();
}
}