无法保存到ViewPager中的SharedPreference

Question

我想在viewpager中保存字符串，在另一个活动中保存SharedPreference。但是，一旦我点击ViewPager中的按钮，程序就会返回错误java.lang.NullPointerException。为什么下面的代码行返回Null Pointer Exception？是因为上下文指向空值吗？

sharedPreference.save(context, test);

以下是ViewPager活动

public class FullScreenImageActivity extends AppCompatActivity {

ViewPager viewPager;
ImageView imageView;
private SharedPreference sharedPreference;

@TargetApi(Build.VERSION_CODES.GINGERBREAD)
public void onCreate(Bundle savedInstanceState) {
    this.requestWindowFeature(Window.FEATURE_NO_TITLE);
    super.onCreate(savedInstanceState);

    setContentView(R.layout.viewpager_image); //TBS

    if (Build.VERSION.SDK_INT > 9) {
        StrictMode.ThreadPolicy policy = new StrictMode.ThreadPolicy.Builder().permitAll().build();
        StrictMode.setThreadPolicy(policy);
    }

    Intent i = getIntent();
    int position = i.getIntExtra("position", 0);
    String[] arr=i.getStringArrayExtra("array");
    String url = i.getStringExtra("url");

    viewPager = (ViewPager) findViewById(R.id.slider);
    viewPager.setAdapter(new GalleryViewPagerAdapter(this, arr,position,url));
    viewPager.setCurrentItem(position);

}


class GalleryViewPagerAdapter extends PagerAdapter {

    private Context context;
    LayoutInflater inflater;
    private String[] imageArrayList;
    private String mUrl;
    private  int mPosition;

    private ProgressDialog mProgress;


    public GalleryViewPagerAdapter(Context _context, String[] imageArrayList, int position, String url) {
        context = _context;
        this.imageArrayList = imageArrayList; //null;
        this.mPosition=position;
        inflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        this.mUrl=url;
    }


    @Override
    public int getCount() {
        return imageArrayList.length;
    }


    @Override
    public Object instantiateItem(final ViewGroup container, final int position) {
        showProgress();

        imageView = (ImageView) findViewById(R.id.btn_favourite);
        imageView.setColorFilter(Color.argb(255, 192, 192, 192));
        imageView.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                //imageView.setColorFilter(Color.argb(255, 249, 0, 0));
            String test="testing";
            sharedPreference.save(context, test);
                Toast.makeText(context,
                        "Added to Favourite!",
                        Toast.LENGTH_SHORT).show();
            }
        });

        View photoRow = inflater.inflate(R.layout.item_image, container,
                false);

        ImageView image = (ImageView) photoRow.findViewById(R.id.img_flickr);

       // added imageloader for better performance
        StaggeredDemoApplication.getImageLoader().get(imageArrayList[position],
                ImageLoader.getImageListener(image, R.drawable.bg_no_image, android.R.drawable.ic_dialog_alert), container.getWidth(), 0);
        ((ViewPager) container).addView(photoRow);
        stopProgress();

        return photoRow;

    }

    private void stopProgress() {

        mProgress.cancel();
    }

    private void showProgress() {
        mProgress = ProgressDialog.show(FullScreenImageActivity.this, "", "Loading...");
    }


    @Override
    public boolean isViewFromObject(View view, Object object) {
        return view == ((RelativeLayout) object);
    }


    @Override
    public void destroyItem(ViewGroup container, int position, Object object) {
        // Remove viewpager_item.xml from ViewPager
        ((ViewPager) container).removeView((RelativeLayout) object);

    }

}
} 

以下是SharedPreference

的程序

public class SharedPreference {

public static final String PREFS_NAME = "AOP_PREFS";
public static final String PREFS_KEY = "AOP_PREFS_String";

public SharedPreference() {
    super();
}

public void save(Context context, String text) {
    SharedPreferences settings;
    Editor editor;

    //settings = PreferenceManager.getDefaultSharedPreferences(context);
    settings = context.getSharedPreferences(PREFS_NAME, Context.MODE_PRIVATE); //1
    editor = settings.edit(); //2

    editor.putString(PREFS_KEY, text); //3

    editor.commit(); //4
}

public String getValue(Context context) {
    SharedPreferences settings;
    String text;

    //settings = PreferenceManager.getDefaultSharedPreferences(context);
    settings = context.getSharedPreferences(PREFS_NAME, Context.MODE_PRIVATE);
    text = settings.getString(PREFS_KEY, null);
    return text;
}

public void clearSharedPreference(Context context) {
    SharedPreferences settings;
    Editor editor;

    //settings = PreferenceManager.getDefaultSharedPreferences(context);
    settings = context.getSharedPreferences(PREFS_NAME, Context.MODE_PRIVATE);
    editor = settings.edit();

    editor.clear();
    editor.commit();
}

public void removeValue(Context context) {
    SharedPreferences settings;
    Editor editor;

    settings = context.getSharedPreferences(PREFS_NAME, Context.MODE_PRIVATE);
    editor = settings.edit();

    editor.remove(PREFS_KEY);
    editor.commit();
}

}