how do I access an already open DHTMLX window

I need to submit a form on a dhtmlx window I've opened with a code-behind javascript.

some code:
window opener in another C# script:

            sbScript.Append(" \n");
            Page.RegisterClientScriptBlock("Progress", sbScript.ToString());


and in another code-behind for Progress.aspx.cs:

                this.lblMessages.Text = "Processing...";
                this.panelProgress.Width = state * 30;
                this.lblPercent.Text = state * 10 + "%";
                Script += "";

                Page.RegisterClientScriptBlock("RefreshWindow", Script);

Javacript error upon running this second snippet:

window.opener is null

whatever I seem to do to access the Form1 on the window and submit it doesn't seem to work:

window.document.Form1.submit();
this.window.Form1.submit();

how can I correctly submit the Form inside the DHTMLX window?

Thanks in advance....