fixed audio settings changed bug

app/src/main/java/org/schabi/newpipe/SettingsActivity.java

@@ -119,6 +119,7 @@ public class SettingsActivity extends PreferenceActivity  {
                 public void onSharedPreferenceChanged(SharedPreferences sharedPreferences,
                                                       String key) {
                     Activity a = getActivity();
+                    if(a != null) {
                         updateSummary();
 
                         if (defaultPreferences.getBoolean(USE_TOR_KEY, false)) {
@@ -133,6 +134,7 @@ public class SettingsActivity extends PreferenceActivity  {
                             App.configureTor(false);
                         }
                     }
+                }
             };
             defaultPreferences.registerOnSharedPreferenceChangeListener(prefListener);