GridView读取SD卡的空指针异常

public class GridViewPics extends Activity {        GridView gridViewMyPics;    Bitmap[] bm;    Bundle bundle=new Bundle();    //fRoot为传进来的文件夹的路径    String fRoots=bundle.getString("fRoot");    @Override    protected void onCreate(Bundle savedInstanceState) {        // TODO Auto-generated method stub        super.onCreate(savedInstanceState);        setContentView(R.layout.grid_view);        gridViewMyPics=(GridView)findViewById(R.id.gridview);        gridViewMyPics.setAdapter(new ImageAdapter(this,getSD()));            }    private List<String> getSD() {        // TODO Auto-generated method stub        List<String> it=new ArrayList<String>();        File f=new File(fRoots); // 就是这里 如果指定一个路径如（"/sdcard/my1"）就是正常的 这样写 就错        File[] files=f.listFiles();        for(int i=0;i<files.length;i++){            File file=files[i];            it.add(file.getPath());        }        return it;    }}