Add missing null check

 Bug: 3417571

Change-Id: I7d3a016eaab53c01532f4743c1f14e57a0b8065b
diff --git a/src/com/android/browser/widget/BookmarkThumbnailWidgetService.java b/src/com/android/browser/widget/BookmarkThumbnailWidgetService.java
index cc06349..e525159 100644
--- a/src/com/android/browser/widget/BookmarkThumbnailWidgetService.java
+++ b/src/com/android/browser/widget/BookmarkThumbnailWidgetService.java
@@ -110,9 +110,11 @@
             if (ids != null) {
                 for (int id : ids) {
                     BookmarkFactory bf = mFactories.remove(id);
-                    // Workaround a known framework bug
-                    // onDestroy is currently never called
-                    bf.onDestroy();
+                    if (bf != null) {
+                        // Workaround a known framework bug
+                        // onDestroy is currently never called
+                        bf.onDestroy();
+                    }
                 }
             }
         } else if (ACTION_CHANGE_FOLDER.equals(action)) {