Android : AlertDialog didn't show up

I create a class that extend base adapter, this adapter will be shown in a fragment. The program run smoothly, but when i clicked an item, it have to show an alertdialog. but the alertdialog didn't show up until i swap to other fragment. This is the class that contain my alertdialog code.

public class GridAdapter extends BaseAdapter {
    private Activity _activity;
    public static ArrayList<String> _filePaths = new ArrayList<String>();
    private ArrayList<String> _imgPaths = new ArrayList<String>();
    private int imageWidth, imageHeight;
    public static ImageButton imageView;
    private PelitaViewer myPage;

    public GridAdapter(Activity activity, ArrayList<String> filePaths, ArrayList<String> imgPaths, int imageWidth, int imageHeight) {
       this._activity = activity;
       this._filePaths = filePaths;
       this._imgPaths = imgPaths;
       this.imageWidth = imageWidth;
       this.imageHeight = imageHeight;

    }

    @Override
    public int getCount(){
        return this._filePaths.size();
    }

    @Override
    public Object getItem(int position){
        return this._filePaths.get(position);
    }

    @Override
    public long getItemId(int position){
        return position;
    }

    @Override
    public View getView(final int position, View convertView, ViewGroup parent){
        if (convertView == null){
            imageView = new ImageButton(_activity);
        } else {
            imageView = (ImageButton) convertView;
        }

        Bitmap image = decodeFile(_imgPaths.get(position), imageWidth, imageHeight);
        imageView.setScaleType(ImageView.ScaleType.CENTER_CROP);
        imageView.setLayoutParams(new GridView.LayoutParams(imageWidth, imageHeight));
        imageView.setImageBitmap(image);
        imageView.setOnClickListener(new OnImageClickListener(position));
        return imageView;
    }
    public static Bitmap decodeFile(String filePath, int WIDTH, int HEIGHT){
        try{
            File f = new File(filePath);
            BitmapFactory.Options o = new BitmapFactory.Options();
            o.inJustDecodeBounds = true;
            BitmapFactory.decodeStream(new FileInputStream(f), null, o);
            final int REQUIRED_WIDTH = WIDTH;
            final int REQUIRED_HEIGHT = HEIGHT;
            int scale = 1;
            while(((o.outWidth/scale/2) > REQUIRED_WIDTH) && ((o.outHeight/scale/2) > REQUIRED_HEIGHT)){
                scale *= 2;
            }
            BitmapFactory.Options o2 = new BitmapFactory.Options();
            o2.inSampleSize = scale;
            return BitmapFactory.decodeStream(new FileInputStream(f), null, o2);
        }catch (FileNotFoundException e){
            e.printStackTrace();
        }
        return null;
    }

    class OnImageClickListener implements OnClickListener{

        int _postion;

        public OnImageClickListener(int position) {
            this._postion = position;
        }

        @Override
        public void onClick(View v) {

            AlertDialog.Builder alert = new AlertDialog.Builder(_activity);
            alert.setTitle("Action!");
            alert.setMessage("What do you want to do?");
            alert.setPositiveButton("OK", null);

            alert.setNegativeButton("Cancel", null);

            alert.show();
        }
    }

}