Bug 887967 - avoid using a null nsIDOMWindowUtils when checking if a window is hidden. r=adw

--HG--
extra : rebase_source : 06c713242b8416839fe4e4a74188f49d014d92bf

toolkit/components/prompts/src/nsPrompter.js

@@ -377,7 +377,7 @@ function openModalWindow(domWin, uri, args) {
         // a domWin was passed, so we can apply the check for it being hidden.
         let winUtils = domWin.QueryInterface(Ci.nsIInterfaceRequestor)
                              .getInterface(Ci.nsIDOMWindowUtils);
-        if (!winUtils.isParentWindowMainWidgetVisible) {
+        if (winUtils && !winUtils.isParentWindowMainWidgetVisible) {
             throw Components.Exception("Cannot call openModalWindow on a hidden window",
                                        Cr.NS_ERROR_NOT_AVAILABLE);
         }