问尝试优化列表视图，但失败

EN

我正在尝试采用Romain Guy去年从google IO推荐的列表ViewHolder来优化列表视图，例如：http://www.softwarepassion.com/android-series-custom-listview-items-and-adapters/ (感谢管理员)，但每当我试图翻阅列表，甚至让它稍微移动一下，应用程序就会崩溃，这是怎么回事？任何帮助都将不胜感激:)这是我所做的>

private class OrderAdapter extends ArrayAdapter<Order> {
    private ArrayList<Order> items;
    LayoutInflater mInflater;

    public OrderAdapter(Context context, int textViewResourceId, ArrayList<Order> items) {
        super(context, textViewResourceId, items);
        mInflater = LayoutInflater.from(context);
        this.items = items;
    }
    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        ViewHolder holder;
        if (convertView == null) {
            convertView = mInflater.inflate(R.layout.row, parent, false);
            holder = new ViewHolder();
            holder.tt = (TextView) convertView.findViewById(R.id.toptext);
            holder.bt = (TextView) convertView.findViewById(R.id.bottomtext);
        } else {
            holder = (ViewHolder) convertView.getTag();
        }
        Order o = items.get(position);
        if (o != null) {
            if (holder.tt != null) {
                holder.tt.setText("Name: " + o.getOrderName());
            }
            if (holder.bt != null) {
                holder.bt.setText("Status: " + o.getOrderStatus());
            }
        }
        return convertView;
    }
}

这是我收到的错误信息，对不起，我不知道如何格式化它>

致命异常: main at com.softberries.lve.SoftwarePassionView$OrderAdapter.getView(SoftwarePassionView.java:98) at android.widget.AbsListView.obtainView(AbsListView.java:1294) at android.widget.ListView.makeAndAddView(ListView.java:1727) at android.widget.ListView.fillDown(ListView.java:652) at android.widget.ListView.fillGap(ListView.java:623) at android.widget.AbsListView.trackMotionScroll(AbsListView.java:2944) at android.widget。AbsListView.onTouchEvent(AbsListView.java:2065)在android.widget.ListView.onTouchEvent(ListView.java:3315)在android.view.View.dispatchTouchEvent(View.java:3766)在android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:897)在android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:936)在android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:936)在android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:936)在android.view.ViewGroupcom.android.internal.policy.impl.PhoneWindow$DecorView.superDispatchTouchEvent(PhoneWindow.java:1671) at com.android.internal.policy.impl.PhoneWindow.superDispatchTouchEvent(PhoneWindow.java:1107) at android.app.Activity.dispatchTouchEvent(Activity.java:2086) at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchTouchEvent(PhoneWindow.java:1655) at android.view.ViewRoot.handleMessage(ViewRoot.java:1785) at android.os.Handler的.dispatchTouchEvent(ViewGroup.java:936).dispatchMessage(Handler.java:99) at android.os.Looper.loop(Looper.java:123) at android.app.ActivityThread.main(ActivityThread.java:4627) at java.lang.reflect.Method.invokeNative(本地方法) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626) (本地方法) at java.lang.reflect.Method.invoke(Method.java:521) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)在dalvik.system.NativeStart.main(本机方法)