Fix crash after detaching document.

Source/core/html/HTMLLabelElement.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2010 Apple Inc. All rights reserved.
  *           (C) 2006 Alexey Proskuryakov (ap@nypop.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
@@ skipping 122 matching lines @@
     return false;
 }
 
 void HTMLLabelElement::defaultEventHandler(Event* evt)
 {
     static bool processingClick = false;
 
     if (evt->type() == EventTypeNames::click && !processingClick) {
         // If text of label element is selected, do not pass
         // the event to control element.
-        if (document().frame()->selection().selection().isRange())
-            return;
+        if (LocalFrame* frame = document().frame()) {
+            if (frame->selection().selection().isRange())
+                return;
+        }
 
         RefPtrWillBeRawPtr<HTMLElement> element = control();
 
         // If we can't find a control or if the control received the click
         // event, then there's no need for us to do anything.
         if (!element || (evt->target() && element->containsIncludingShadowDOM(evt->target()->toNode())))
             return;
 
         if (evt->target() && isInInteractiveContent(evt->target()->toNode()))
             return;
@@ skipping 34 matching lines @@
 
 void HTMLLabelElement::accessKeyAction(bool sendMouseEvents)
 {
     if (HTMLElement* element = control())
         element->accessKeyAction(sendMouseEvents);
     else
         HTMLElement::accessKeyAction(sendMouseEvents);
 }
 
 } // namespace