Fix AppModalDialogHelper to deal with a TabStrip without an active tab

chrome/browser/ui/blocked_content/app_modal_dialog_helper.cc

 // Copyright 2014 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/browser/ui/blocked_content/app_modal_dialog_helper.h"
 
 #include "chrome/browser/ui/browser.h"
 #include "chrome/browser/ui/browser_finder.h"
 #include "chrome/browser/ui/browser_list.h"
 #include "chrome/browser/ui/tabs/tab_strip_model.h"
 #include "content/public/browser/web_contents.h"
 #include "content/public/browser/web_contents_delegate.h"
 
 AppModalDialogHelper::AppModalDialogHelper(content::WebContents* dialog_host)
     : popup_(nullptr) {
   // If the WebContents that triggered this dialog is not currently focused, we
   // want to store a potential popup here to restore it after the dialog was
   // closed.
   chrome::HostDesktopType desktop_type =
       chrome::GetHostDesktopTypeForNativeView(dialog_host->GetNativeView());
   Browser* active_browser =
       BrowserList::GetInstance(desktop_type)->GetLastActive();
   content::WebContents* active_web_contents =
       active_browser->tab_strip_model()->GetActiveWebContents();
   if (active_browser->is_type_popup() &&
+      active_web_contents &&
       active_web_contents->GetOpener() == dialog_host) {
     // It's indeed a popup from the dialog opening WebContents. Store it, so
     // we can focus it later.
     popup_ = active_web_contents;
     Observe(popup_);
   }
 }
 
 AppModalDialogHelper::~AppModalDialogHelper() {
   if (popup_)
     popup_->GetDelegate()->ActivateContents(popup_);
 }
 
 void AppModalDialogHelper::WebContentsDestroyed() {
   popup_ = nullptr;
 }