NTP checks for guest sessions before updating the login bar.

chrome/browser/ui/webui/ntp/ntp_login_handler.cc

Index: chrome/browser/ui/webui/ntp/ntp_login_handler.cc
diff --git a/chrome/browser/ui/webui/ntp/ntp_login_handler.cc b/chrome/browser/ui/webui/ntp/ntp_login_handler.cc
index c53b1ba7873a880000f211d1f8102d4f53c46b1e..98963923a60d7c3bc872c51eaae36cc6703a1081 100644
--- a/chrome/browser/ui/webui/ntp/ntp_login_handler.cc
+++ b/chrome/browser/ui/webui/ntp/ntp_login_handler.cc
@@ -173,6 +173,9 @@ void NTPLoginHandler::HandleShowAdvancedLoginUI(const base::ListValue* args) {
 
 void NTPLoginHandler::UpdateLogin() {
   Profile* profile = Profile::FromWebUI(web_ui());
+  if (profile->IsGuestSession())
+    return;
+
   std::string username =
       SigninManagerFactory::GetForProfile(profile)->GetAuthenticatedUsername();

[Dan Beam, 2015/03/18 17:08:09]

auto* factory = SigninManagerFactory::GetForProfile(profile);
if (!factory) {
  // Happens in guest mode.
  return;
}

[Mike Lerman, 2015/03/18 19:23:47]

Done.