--- a/calendar/itip/calItipEmailTransport.js
+++ b/calendar/itip/calItipEmailTransport.js
@@ -187,17 +187,21 @@ calItipEmailTransport.prototype = {
                 cal.LOG("sendXpcomMail: Found USER autoResponse type.\n" +
                         "This type is currently unsupported, the compose API will always enter a text/plain\n" +
                         "or text/html part as first part of the message.\n" +
                         "This will disable OL (up to 2003) to consume the mail as an iTIP invitation showing\n" +
                         "the usual calendar buttons.");
                 // To somehow have a last resort before sending spam, the user can choose to send the mail.
                 let prefCompatMode = cal.getPrefSafe("calendar.itip.compatSendMode", 0);
                 let inoutCheck = { value: (prefCompatMode == 1) };
-                if (Services.prompt.confirmEx(null,
+                let parent = Services.wm.getMostRecentWindow(null);
+                if (parent.closed) {
+                    parent = cal.getCalendarWindow();
+                }
+                if (Services.prompt.confirmEx(parent,
                                               cal.calGetString("lightning", "imipSendMail.title", null, "lightning"),
                                               cal.calGetString("lightning", "imipSendMail.text", null, "lightning"),
                                               Services.prompt.STD_YES_NO_BUTTONS,
                                               null,
                                               null,
                                               null,
                                               cal.calGetString("lightning", "imipSendMail.Outlook2000CompatMode.text", null, "lightning"),
                                               inoutCheck)) {