How correctly save game progress via libGDX Preferences on Android devices?

Question

I am using next code. This code work fine on desktop version, but on Android device does not. return Gdx.app.getPreferences(PREFS_NAME) always is null. Why? Where I could make mistake?

import com.badlogic.gdx.Gdx;
import com.badlogic.gdx.Preferences;

public class BeatlesPreferences {
    // constants
    private static final String PREF_VIBRO = "vibro";
    private static final String PREF_MUSIC_ENABLED = "musicenabled";
    private static final String PREF_SOUND_ENABLED = "soundenabled";
    private static final String PREFS_NAME = "my_app";

    public BeatlesPreferences() {
    }

    protected Preferences getPrefs() {
        return Gdx.app.getPreferences(PREFS_NAME);
    }

    public boolean isSoundEffectsEnabled() {
        return getPrefs().getBoolean(PREF_SOUND_ENABLED, true);
    }

    public void setSoundEffectsEnabled(boolean soundEffectsEnabled) {
        getPrefs().putBoolean(PREF_SOUND_ENABLED, soundEffectsEnabled);
        getPrefs().flush();
    }

    public boolean isMusicEnabled() {
        return getPrefs().getBoolean(PREF_MUSIC_ENABLED, true);
    }

    public void setMusicEnabled(boolean musicEnabled) {  
        getPrefs().putBoolean(PREF_MUSIC_ENABLED, musicEnabled);
        getPrefs().flush();
    }

    public boolean isVibroEnabled() {
        return getPrefs().getBoolean(PREF_VIBRO, true);
    }

    public void setVibroEnabled(boolean vibro) {
        getPrefs().putBoolean(PREF_VIBRO, vibro);
        getPrefs().flush();
    }
}

Answer 1

Use like this:

   private Preferences preferences;
   protected Preferences getPrefs() {
      if(preferences==null){
         preferences = Gdx.app.getPreferences(PREFS_NAME);
      }
   return preferences;
   }