fix NullPointerException

[Liloveyang]

Hello,Thank you in advance for making your code public.
The exception comes from the drawBitmap method under the MainSurface class！

     @Override
      public void drawBitmap(Bitmap bitmap, Matrix matrix, Paint paint) {	
          this.paint.getGraphics().drawImage(bitmap.getImage(),((JMatrix)matrix).trans, null);
      }

When executing bitmap.getImage (), NullPointerException is caused because the bitmap is null. You should add the if module! Thank you for reading