Bug 1072342, the child process doesn't have a main widget, r=smaug

dom/base/nsGlobalWindow.cpp

@@ -9824,14 +9824,14 @@ nsGlobalWindow::ActivateOrDeactivate(bool aActivate)
   // Set / unset mIsActive on the top level window, which is used for the
   // :-moz-window-inactive pseudoclass, and its sheet (if any).
   nsCOMPtr<nsIWidget> mainWidget = GetMainWidget();
-  if (!mainWidget)
-    return;
-
-  // Get the top level widget (if the main widget is a sheet, this will
-  // be the sheet's top (non-sheet) parent).
-  nsCOMPtr<nsIWidget> topLevelWidget = mainWidget->GetSheetWindowParent();
-  if (!topLevelWidget) {
-    topLevelWidget = mainWidget;
+  nsCOMPtr<nsIWidget> topLevelWidget;
+  if (mainWidget) {
+    // Get the top level widget (if the main widget is a sheet, this will
+    // be the sheet's top (non-sheet) parent).
+    topLevelWidget = mainWidget->GetSheetWindowParent();
+    if (!topLevelWidget) {
+      topLevelWidget = mainWidget;
+    }
   }
 
   nsCOMPtr<nsPIDOMWindow> piMainWindow(