scrollView返回null位图的屏幕截图

Question

我正在尝试拍摄滚动视图的所有内容（也在页面外）。我遇到的问题是，当它到达行Bitmap b = u.getDrawingCache();时，抛出异常“nullPointerException”。我似乎无法弄清楚造成这种情况的原因。这是我的代码：

if(v.getId() == R.id.btnSnap)
    {
        try
        {
            View u = findViewById(R.id.svContent);
            u.setDrawingCacheEnabled(true);
            ScrollView z = (ScrollView) findViewById(R.id.svContent);
            int totalHeight = z.getChildAt(0).getHeight();
            int totalWidth = z.getChildAt(0).getWidth();
            u.layout(0, 0, totalWidth, totalHeight);
            u.buildDrawingCache(true);
            Bitmap b = u.getDrawingCache();

            //Just so that I can get a quick preview of 
            //what the picture looks like
            ImageView img = (ImageView)findViewById(R.id.ivTest);
            img.setImageBitmap(b);

            //Save the picture
            File fileName = new File(context.getExternalFilesDir(null)+"/ArcFlash/SafetyCheckList1.png");
            FileOutputStream out = new FileOutputStream(fileName);
            b.compress(Bitmap.CompressFormat.PNG, 90, out);
            out.close();
        }
        catch(Exception ex)
        {
            handler.Log(ex.getMessage(), "onClick(SafetyCheckList.java)", context);
        }
    }

编辑问题似乎在u.layout(0, 0, totalWidth, totalHeight);