对话框抛出“无法添加窗口-令牌null不适用于应用程序”

我的代码：

    public class MainActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        NoticeBoard.init(MainActivity.this);
        NoticeBoard.getInstance().setButton();
        setContentView(NoticeBoard.getInstance());
    }

}

    public class NoticeBoard extends ImageView {
    private static NoticeBoard instance = null;
    private static Paint paint;
    protected  Context mContext;
    private static WindowManager mWindowManager;

    public NoticeBoard(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    public static NoticeBoard getInstance() {
        return instance;
    }

    public static void init(Context context) {
        instance = new NoticeBoard(context);
        instance.setImageResource(R.drawable.ic_launcher);
        instance.invalidate();
    }

    public NoticeBoard(Context context) {
        super(context);
        mWindowManager = (WindowManager) getContext().getApplicationContext().getSystemService(Context.WINDOW_SERVICE); 
        mContext = context;
        setBackgroundColor(Color.BLACK);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        paint = new Paint(Paint.LINEAR_TEXT_FLAG);
        paint.setColor(Color.GRAY);
        paint.setTextSize(12.0F);
        canvas.drawText("Hello World", 100, 200, paint);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        return super.onTouchEvent(event);
    }

    public void setButton() {
        Activity a =(Activity) mContext;
        ImageButton btn = new ImageButton(a);
        btn.setBackgroundResource(R.drawable.ic_launcher);
        android.view.WindowManager.LayoutParams paramsSplit = new android.view.WindowManager.LayoutParams();
        paramsSplit.width=2;
        paramsSplit.height=2;
        instance.mWindowManager.addView(btn, paramsSplit);
    }
}

我试图找到答案，我将getApplicationContext（）更改为MainActivity.this，并将上下文更改为（Activity）Context，但仍然无法正常工作。它出什么问题了？主席：我是中文，我的英语不太好。谢谢你的帮助。