Fix widget stopListening NPE.

lookupHostLocked caller needs to check for null return value.
diff --git a/services/java/com/android/server/AppWidgetService.java b/services/java/com/android/server/AppWidgetService.java
index 5439f8b..3c46954 100644
--- a/services/java/com/android/server/AppWidgetService.java
+++ b/services/java/com/android/server/AppWidgetService.java
@@ -473,8 +473,10 @@
     public void stopListening(int hostId) {
         synchronized (mAppWidgetIds) {
             Host host = lookupHostLocked(getCallingUid(), hostId);
-            host.callbacks = null;
-            pruneHostLocked(host);
+            if (host != null) {
+                host.callbacks = null;
+                pruneHostLocked(host);
+            }
         }
     }