CHECK that the frames that UserScriptScheduler gets are non-NULL. We're seeing

chrome/renderer/extensions/user_script_scheduler.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/renderer/extensions/user_script_scheduler.h"
 
 #include "base/bind.h"
 #include "base/logging.h"
 #include "base/message_loop/message_loop.h"
 #include "chrome/renderer/chrome_render_process_observer.h"
@@ skipping 153 matching lines @@
   frame_vector.push_back(frame_);
   if (params.all_frames)
     GetAllChildFrames(frame_, &frame_vector);
 
   std::string error;
 
   scoped_ptr<blink::WebScopedUserGesture> gesture;
   if (params.user_gesture)
     gesture.reset(new blink::WebScopedUserGesture);
 
+  GURL top_url = frame_->document().url();
+
   for (std::vector<WebFrame*>::iterator frame_it = frame_vector.begin();
        frame_it != frame_vector.end(); ++frame_it) {
     WebFrame* child_frame = *frame_it;
+    CHECK(child_frame) << top_url;
+
     // We recheck access here in the renderer for extra safety against races
     // with navigation.
     //
     // But different frames can have different URLs, and the extension might
     // only have access to a subset of them. For the top frame, we can
     // immediately send an error and stop because the browser process
     // considers that an error too.
     //
     // For child frames, we just skip ones the extension doesn't have access
     // to and carry on.
     if (!params.is_web_view &&
         !PermissionsData::CanExecuteScriptOnPage(extension,
                                                  child_frame->document().url(),
-                                                 frame_->document().url(),
+                                                 top_url,
                                                  extension_helper->tab_id(),
                                                  NULL,
                                                  -1,
                                                  NULL)) {
       if (child_frame->parent()) {
         continue;
       } else {
         error = ErrorUtils::FormatErrorMessage(
             manifest_errors::kCannotAccessPage,
             child_frame->document().url().spec());
@@ skipping 63 matching lines @@
 
   for (WebFrame* child_frame = parent_frame->firstChild(); child_frame;
        child_frame = child_frame->nextSibling()) {
     frames_vector->push_back(child_frame);
     GetAllChildFrames(child_frame, frames_vector);
   }
   return true;
 }
 
 }  // namespace extensions