Fix NPE.

I believe this is the result of a race condition involving multiple tickers under load. The fix simply guards against the null pointer.

Bug: 3448450
Change-Id: Icc78c89890f0a41c574f7077b1b450e9fd4ca28b

packages/SystemUI/src/com/android/systemui/statusbar/tablet/TabletTicker.java

@@ -158,7 +158,9 @@ public class TabletTicker
     private void advance() {
         // Out with the old...
         if (mCurrentView != null) {
-            mWindow.removeView(mCurrentView);
+            if (mWindow != null) {
+                mWindow.removeView(mCurrentView);
+            }
             mCurrentView = null;
             mCurrentKey = null;
             mCurrentNotification = null;