Only the main document should have a StyleEngine.

sky/engine/core/page/Page.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013 Apple Inc. All Rights Reserved.
  * Copyright (C) 2008 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  * This library is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 205 matching lines @@
             (*it)->multisamplingChanged(m_settings->openGLMultisamplingEnabled());
         break;
     }
     case SettingsDelegate::ImageLoadingChange:
         if (mainFrame() && mainFrame()->document()) {
             mainFrame()->document()->fetcher()->setImagesEnabled(settings().imagesEnabled());
             mainFrame()->document()->fetcher()->setAutoLoadImages(settings().loadsImagesAutomatically());
         }
         break;
     case SettingsDelegate::FontFamilyChange:
-        if (mainFrame()->document())
+        if (mainFrame()->document() && mainFrame()->document()->isActive())
             mainFrame()->document()->styleEngine()->updateGenericFontFamilySettings();
         setNeedsRecalcStyleInAllFrames();
         break;
     case SettingsDelegate::AcceleratedCompositingChange:
         // FIXME(sky): Remove
         break;
     case SettingsDelegate::MediaQueryChange:
         if (mainFrame()->document())
             mainFrame()->document()->mediaQueryAffectingValueChanged();
         setNeedsRecalcStyleInAllFrames();
@@ skipping 45 matching lines @@
     , editorClient(0)
     , spellCheckerClient(0)
 {
 }
 
 Page::PageClients::~PageClients()
 {
 }
 
 } // namespace blink