android简单开发，使用bmob作为服务器后端，上传图片到bmob失败

以下为我上传的代码：
public void storePicture(){
        File file=new File("sdcard/"+PATH);
        if (file!=null){
            final BmobFile bmobFile=new BmobFile(file);
            bmobFile.uploadblock(new UploadFileListener() {
                @Override
                public void done(BmobException e) {
                    if (e==null){
                        Picture picture=new Picture();
                        picture.setContent(textView.getText().toString());
                        picture.setFile(bmobFile);
                        picture.save(new SaveListener<String>() {
                            @Override
                            public void done(String s, BmobException e) {
                                if (e == null) {
                                    Toast.makeText(MainActivity.this, "存取图片成功", Toast.LENGTH_SHORT).show();
                                } else {
                                    Toast.makeText(MainActivity.this, "存取失败3", Toast.LENGTH_SHORT).show();
                                }
                            }
                        });
                    }else{
                        Toast.makeText(MainActivity.this,"存取失败2",Toast.LENGTH_SHORT).show();
                    }
                }
            });
        }    }
然后每次点击上传按钮后，都会提示错误：
java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context cn.bmob.v3.BmobWrapper.getApplicationContext()' on a null object reference
以下是完整的代码，主要就是实现一个简单的点击添加按钮，能够获取手机图片，然后点击上传按钮，能够上传图片到bmob上：public class MainActivity extends Activity implements View.OnClickListener{
    private TextView textView;
    private ImageView imageView;
    private Button btnAdd,btnSave;
    final private int GET_PICTURE=1;
    final private int SAVE_PICTURE=2;
    private String PATH;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        initView();
        btnSave.setOnClickListener(this);
        btnAdd.setOnClickListener(this);
    }
    public void initView(){
        textView=(TextView)findViewById(R.id.text);
        imageView=(ImageView)findViewById(R.id.image);
        btnAdd=(Button)findViewById(R.id.addImage);
        btnSave=(Button)findViewById(R.id.saveImage);
    }    @Override
    public void onClick(View v) {
        switch (v.getId()){
            //获取本地图片,并显示
            case R.id.addImage:
                getPicture();
                break;
            //上传图片
            case R.id.saveImage:
                storePicture();
                break;
        }
    }
    public void getPicture(){
        Intent intent=new Intent();
        intent.setType("image/*");
        intent.setAction(Intent.ACTION_GET_CONTENT);
        startActivityForResult(intent,GET_PICTURE);
    }    public void storePicture(){
        File file=new File("sdcard/"+PATH);
        if (file!=null){
            final BmobFile bmobFile=new BmobFile(file);
            bmobFile.uploadblock(new UploadFileListener() {
                @Override
                public void done(BmobException e) {
                    if (e==null){
                        Picture picture=new Picture();
                        picture.setContent(textView.getText().toString());
                        picture.setFile(bmobFile);
                        picture.save(new SaveListener<String>() {
                            @Override
                            public void done(String s, BmobException e) {
                                if (e == null) {
                                    Toast.makeText(MainActivity.this, "存取图片成功", Toast.LENGTH_SHORT).show();
                                } else {
                                    Toast.makeText(MainActivity.this, "存取失败3", Toast.LENGTH_SHORT).show();
                                }
                            }
                        });
                    }else{
                        Toast.makeText(MainActivity.this,"存取失败2",Toast.LENGTH_SHORT).show();
                    }
                }
            });
        }    }
    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);
        switch (requestCode){
            case GET_PICTURE:
                if (data!=null){
                    Uri selectedImageUri=data.getData();
                    String[] proj={MediaStore.Images.Media.DATA};
                    Cursor cursor=managedQuery(selectedImageUri,proj,null,null,null);
                    int column_index=cursor.getColumnIndexOrThrow(MediaStore.Images.Media.DATA);
                    cursor.moveToFirst();
                    PATH=cursor.getString(column_index);
                    textView.setText(PATH);
                    try{
                        Bitmap bitmap= BitmapFactory.decodeStream(this.getContentResolver().openInputStream(Uri.parse(selectedImageUri.toString())));
                        imageView.setImageBitmap(bitmap);
                    }catch (FileNotFoundException e){
                        e.printStackTrace();
                    }
                }else {
                    Toast.makeText(this,"获取图片失败",Toast.LENGTH_SHORT).show();
                }
                break;
        }
    }}
求大神们帮忙解决下这个问题，debug了好久，也没找出来！