Android 当我尝试选择所有复选框时，为什么我的复选框变为空

我知道这个问题很熟悉，但我需要帮助。我不知道为什么我的复选框变成了空指针。到目前为止，我已经做到了：

public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_gallery_image);

    final String[] columns = { MediaStore.Images.Media.DATA, MediaStore.Images.Media._ID };
    final String orderBy = MediaStore.Images.Media._ID;
    Cursor imagecursor = managedQuery(
            MediaStore.Images.Media.EXTERNAL_CONTENT_URI, columns, null,
            null, orderBy);
    int image_column_index = imagecursor.getColumnIndex(MediaStore.Images.Media._ID);
    this.count = imagecursor.getCount();
    this.thumbnails = new Bitmap[this.count];
    this.arrPath = new String[this.count];
    this.thumbnailsselection = new boolean[this.count];
    for (int i = 0; i < this.count; i++) {
        imagecursor.moveToPosition(i);
        int id = imagecursor.getInt(image_column_index);
        int dataColumnIndex = imagecursor.getColumnIndex(MediaStore.Images.Media.DATA);
        thumbnails[i] = MediaStore.Images.Thumbnails.getThumbnail(
                getApplicationContext().getContentResolver(), id,
                MediaStore.Images.Thumbnails.MICRO_KIND, null);
        arrPath[i]= imagecursor.getString(dataColumnIndex);
    }
    final GridView imagegrid = (GridView) findViewById(R.id.grid_GalleryImage);
    imageAdapter = new ImageAdapter();
    imagegrid.setAdapter(imageAdapter);

    // Check All

    Button btnCheckAll = (Button) findViewById(R.id.btn_SelectAll);
    btnCheckAll.setOnClickListener(new OnClickListener() {
            public void onClick(View v) {
                int count = thumbnails.length;
                for (int i = 0; i < count; i++) {
                    LinearLayout itemLayout = (LinearLayout)imagegrid.getChildAt(i); // Find by under LinearLayout
                    CheckBox checkbox = (CheckBox)itemLayout.findViewById(R.id.cbo_CheckImage);
                    checkbox.setChecked(true);
                } 

            }
    });

---

“为什么我的复选框变成空指针”是什么意思？请详细说明，它不清楚。在我的btncheckall.setonclicklistener复选框中，他们返回空指针，我的代码有问题。

public class ImageAdapter extends BaseAdapter {
    private LayoutInflater mInflater;
    ViewHolder holder;
    public ImageAdapter() {
        mInflater = (LayoutInflater) getSystemService(Context.LAYOUT_INFLATER_SERVICE);

    }

    public int getCount() {
        return count;
    }

    public Object getItem(int position) {
        return position;
    }

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

    public View getView(int position, View convertView, ViewGroup parent) {


        if (convertView == null) {
            holder = new ViewHolder();
            convertView = mInflater.inflate(
                    R.layout.gallery, null);


            holder.imageview = (ImageView) convertView.findViewById(R.id.img_thumbImage);
            holder.checkbox = (CheckBox) convertView.findViewById(R.id.cbo_CheckImage);


            convertView.setTag(holder);
        }
        else {
            holder = (ViewHolder) convertView.getTag();
        }
        holder.checkbox.setId(position);
        holder.imageview.setId(position);

        holder.checkbox.setOnClickListener(new OnClickListener() {

            public void onClick(View v) {
                // TODO Auto-generated method stub
                CheckBox cb = (CheckBox) v;
                int id = cb.getId();
                if (thumbnailsselection[id]){
                    cb.setChecked(false);
                    thumbnailsselection[id] = false;
                } else {
                    cb.setChecked(true);
                    thumbnailsselection[id] = true;
                }
            }
        });
        holder.imageview.setOnClickListener(new OnClickListener() {

            public void onClick(View v) {
                // TODO Auto-generated method stub
                int id = v.getId();
                Intent intent = new Intent();
                intent.setAction(Intent.ACTION_VIEW);
                intent.setDataAndType(Uri.parse("file://" + arrPath[id]), "image/*");
                startActivity(intent);
            }
        });
        holder.imageview.setImageBitmap(thumbnails[position]);
        holder.checkbox.setChecked(thumbnailsselection[position]);
        holder.id = position;
        return convertView;
    }
}
class ViewHolder {
    CheckBox checkbox;
    ImageView imageview;

    int id;
}