sync: fix tiny bug with NotificationServiceSessionsRouter

chrome/browser/sync/sessions2/notification_service_sessions_router.cc

Index: chrome/browser/sync/sessions2/notification_service_sessions_router.cc
diff --git a/chrome/browser/sync/sessions2/notification_service_sessions_router.cc b/chrome/browser/sync/sessions2/notification_service_sessions_router.cc
index 084589078dc4cf2268441cf1f878969c40466fe4..7c96c6432d7801e55acd4294348127e9cfdfd3d2 100644
--- a/chrome/browser/sync/sessions2/notification_service_sessions_router.cc
+++ b/chrome/browser/sync/sessions2/notification_service_sessions_router.cc
@@ -135,7 +135,7 @@ void NotificationServiceSessionsRouter::OnNavigationBlocked(
     content::WebContents* web_contents) {
   SyncedTabDelegate* tab =
       SyncedTabDelegate::ImplFromWebContents(web_contents);
-  if (!tab)
+  if (!tab || !handler_)
     return;
 
   DCHECK(tab->profile() == profile_);