Fix NPE

Check whether SMS delivery report preference is available first because
it could be removed if carriers don't support it.

Test: Manual

Signed-off-by: Taesu Lee <taesu82.lee@samsung.com>
Change-Id: I9e0049dd61713f7a9638fb19a37aaa261f35d768

src/com/android/messaging/ui/appsettings/PerSubscriptionSettingsActivity.java

@@ -183,7 +183,9 @@ public class PerSubscriptionSettingsActivity extends BugleActionBarActivity {
                 autoRetrieveMmsPreference.setEnabled(false);
                 final Preference deliveryReportsPreference =
                         findPreference(getString(R.string.delivery_reports_pref_key));
-                deliveryReportsPreference.setEnabled(false);
+                if (deliveryReportsPreference != null) {
+                    deliveryReportsPreference.setEnabled(false);
+                }
             }
         }