Check for null activity before getting dashboard categories.

In DashboardSummary, when async task finishes running, ensure that the
fragment is still associated with the activity before getting the
dashboard categories from the activity.

Change-Id: If23cd27a0d3e3429d900ae41ef6f24ed03b9e8d6
Fixes: 30278854

src/com/android/settings/dashboard/DashboardSummary.java

@@ -16,6 +16,7 @@
 
 package com.android.settings.dashboard;
 
+import android.app.Activity;
 import android.content.Context;
 import android.os.AsyncTask;
 import android.os.Bundle;
@@ -231,8 +232,12 @@ public class DashboardSummary extends InstrumentedFragment
 
         @Override
         protected void onPostExecute(List<Tile> tiles) {
+            final Activity activity = getActivity();
+            if (activity == null) {
+                return;
+            }
             List<DashboardCategory> categories =
-                    ((SettingsActivity) getActivity()).getDashboardCategories();
+                    ((SettingsActivity) activity).getDashboardCategories();
             mAdapter.setCategoriesAndSuggestions(categories, tiles);
         }
     }