AirplaneModePreferenceController: Fix NPE when toggle is hidden

When airplane mode toggle is not available, i.e. the overlay
config_show_toggle_airplane is set to false, the Settings app
will crash when leaving "Network and Internet" menu.

Change-Id: Iaf5bd05d59091ba615a55eedc5243806a627da7b

src/com/android/settings/network/AirplaneModePreferenceController.java

@@ -141,7 +141,9 @@ public class AirplaneModePreferenceController extends TogglePreferenceController
 
     @Override
     public void onDestroy() {
-        mAirplaneModeEnabler.close();
+        if (isAvailable()) {
+            mAirplaneModeEnabler.close();
+        }
     }