在自定义ListView适配器中执行findViewById()时发生NullPointerException

public class CustomAdapter extends BaseAdapter {
    private List<Item> itemList;
    private LayoutInflater inflater;

    public CustomAdapter(Context context, List<Item> itemList) {
        this.itemList = itemList;
        inflater = LayoutInflater.from(context);
    }

    @Override
    public int getCount() {
        return itemList.size();
    }

    @Override
    public Object getItem(int position) {
        return itemList.get(position);
    }

    @Override
    public long getItemId(int position) {
        return position;
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        ViewHolder viewHolder;

        if (convertView == null) {
            convertView = inflater.inflate(R.layout.list_item, parent, false);
            viewHolder = new ViewHolder();
            viewHolder.textView = convertView.findViewById(R.id.text_view);
            convertView.setTag(viewHolder);
        } else {
            viewHolder = (ViewHolder) convertView.getTag();
        }

        Item item = itemList.get(position);
        viewHolder.textView.setText(item.getText());

        return convertView;
    }

    private static class ViewHolder {
        TextView textView;
    }
}