Fix a Deletion Helper crash when it fails to get an app list.

Bug 28424827
Change-Id: I5c3c426d557fb1647a3909bf88fbb7d24329dc57

src/com/android/settings/deletionhelper/DeletionHelperFragment.java

@@ -153,6 +153,7 @@ public class DeletionHelperFragment extends SettingsPreferenceFragment implement
         final ArrayList<AppEntry> apps =
                 mSession.rebuild(AppStateUsageStatsBridge.FILTER_USAGE_STATS,
                         ApplicationsState.SIZE_COMPARATOR);
+        if (apps == null) return;
         mAppEntries = apps;
         cacheRemoveAllPrefs(mApps);
         int entryCount = apps.size();