Adding the system context menu to right click event on item

Question

I've been looking for a way to add the system context menu to my own item, and the only way i found is to make custom context menu. It works but the design is not like the default menu. Can someone help me with this.

Here is the code i tried:

private void topPanel_MouseDown(object sender, MouseEventArgs e)
        {
            if (e.Button == MouseButtons.Left)
            {
                ReleaseCapture();
                SendMessage(Handle, WM_NCLBUTTONDOWN, HT_CAPTION, 0);
            }
            else if (e.Button == MouseButtons.Right)
            {
                ContextMenu cm = this.ContextMenu;
                cm.Show(topPanel, new System.Drawing.Point(20, 20));
            }
        }

At first look it work and doesn't show any error, but when i right click on the item it shows me this error: Object reference not set to an instance of an object.