added null check in doInBackground and if null, properly intializes

variables.

Bug:29234237
Change-Id: I1d65b1a232c71efdd11a917769aec5044867a085
(cherry picked from commit 0f0f5414f4)

packages/SettingsLib/src/com/android/settingslib/drawer/SettingsDrawerActivity.java

@@ -307,6 +307,13 @@ public class SettingsDrawerActivity extends Activity {
             return TileUtils.getCategories(SettingsDrawerActivity.this, sTileCache);
         }
 
+        @Override
+        protected void onPreExecute() {
+            if (sConfigTracker == null || sTileCache == null) {
+                getDashboardCategories();
+            }
+        }
+
         @Override
         protected void onPostExecute(List<DashboardCategory> dashboardCategories) {
             for (int i = 0; i < dashboardCategories.size(); i++) {