private void barButtonItem4_ItemClick(object sender, DevExpress.XtraBars.ItemClickEventArgs e)
{
KhachHangGUI f = new KhachHangGUI();
f.MdiParent = this;
f.Show();
}
private void btnKhachHang_ItemClick(object sender, ItemClickEventArgs e)
{
KhachHangGUI khachHangGUI = new KhachHangGUI();
AddTabControl(khachHangGUI, "Khách hàng");
}