Fixing drag and drop visibility issues of tabs on a visiting desktop

chrome/browser/ui/ash/launcher/browser_status_monitor.cc

 // Copyright 2013 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/ash/launcher/browser_status_monitor.h"
 
 #include "ash/shell.h"
 #include "ash/wm/window_util.h"
 #include "base/stl_util.h"
 #include "chrome/browser/ui/ash/launcher/browser_shortcut_launcher_item_controller.h"
@@ skipping 79 matching lines @@
   }
 
   STLDeleteContainerPairSecondPointers(webcontents_to_observer_map_.begin(),
                                        webcontents_to_observer_map_.end());
 }
 
 void BrowserStatusMonitor::UpdateAppItemState(
     content::WebContents* contents,
     ChromeLauncherController::AppState app_state) {
   DCHECK(contents);
-  if (launcher_controller_->IsBrowserFromActiveUser(
-          chrome::FindBrowserWithWebContents(contents)))
+  // It is possible to come here from Browser::SwapTabContent where the contents
+  // cannot be associated with a browser.
+  Browser* browser = chrome::FindBrowserWithWebContents(contents);
+  if (browser && launcher_controller_->IsBrowserFromActiveUser(browser))
     launcher_controller_->UpdateAppState(contents, app_state);
 }
 
 void BrowserStatusMonitor::UpdateBrowserItemState() {
   launcher_controller_->GetBrowserShortcutLauncherItemController()->
       UpdateBrowserItemState();
 }
 
 void BrowserStatusMonitor::OnWindowActivated(aura::Window* gained_active,
                                              aura::Window* lost_active) {
@@ skipping 194 matching lines @@
   }
 }
 
 void BrowserStatusMonitor::RemoveWebContentsObserver(
     content::WebContents* contents) {
   DCHECK(webcontents_to_observer_map_.find(contents) !=
       webcontents_to_observer_map_.end());
   delete webcontents_to_observer_map_[contents];
   webcontents_to_observer_map_.erase(contents);
 }