Fix crash in screensaver shutdown.

chrome/browser/chromeos/kiosk_mode/kiosk_mode_screensaver.cc

Index: chrome/browser/chromeos/kiosk_mode/kiosk_mode_screensaver.cc
diff --git a/chrome/browser/chromeos/kiosk_mode/kiosk_mode_screensaver.cc b/chrome/browser/chromeos/kiosk_mode/kiosk_mode_screensaver.cc
index 9e1b7552c5057007d2c07ded6fc964a745a9807b..1daa428bab826254ed525cc130b127e838aef76b 100644
--- a/chrome/browser/chromeos/kiosk_mode/kiosk_mode_screensaver.cc
+++ b/chrome/browser/chromeos/kiosk_mode/kiosk_mode_screensaver.cc
@@ -127,7 +127,9 @@ KioskModeScreensaver::~KioskModeScreensaver() {
 
   // In case we're shutting down without ever triggering the active
   // notification and/or logging in.
-  ash::Shell::GetInstance()->user_activity_detector()->RemoveObserver(this);
+  if (ash::Shell::GetInstance() &&
+      ash::Shell::GetInstance()->user_activity_detector())
+    ash::Shell::GetInstance()->user_activity_detector()->RemoveObserver(this);
   registrar_.RemoveAll();
 }