Android ListView适配器崩溃问题/重复项

我基本上是想通过同一ListView适配器显示多个视图。但是，适配器最终会生成多个重复项，并且有时也会因NullPointer而崩溃。我的猜测是我完全错误地实现了适配器。这是我完整的代码：

该项目可以是照片或文字。

适配器：

 public class FeedAdapter extends BaseAdapter {

        static private Activity activity;
        private static LayoutInflater inflater = null;
        ArrayList<ActivityTable> actList = new ArrayList<ActivityTable>();
        Holder holder;

    public FeedAdapter(Activity a, ArrayList<ActivityTable> actList) {
            activity = a;
            this.actList = actList;
        }

    public View getView(int position, View convertView, ViewGroup parent) {

            Holder holder;

            final ActivityTable act = actList.get(position);
    inflater = (LayoutInflater) activity.getSystemService(Context.LAYOUT_INFLATER_SERVICE);

     if (convertView == null) {

                if (act.getType().equals("text")) {

                    convertView = inflater.inflate(R.layout.feed_single_text, null);
                    holder = new Holder();

                    //More code that Set the caption to the holder
                    convertView.setTag(holder);

                }

                if (act.getType().equals("photo")) {

                    convertView = inflater.inflate(R.layout.feed_single_picture, parent, false);
                    holder = new Holder();
                    holder.media = (ImageView) convertView.findViewById(R.id.postphoto);
                    //More code that Set the photo to the holder
                    convertView.setTag(holder);
                }

            } else {

                holder = (Holder) convertView.getTag();

            }

         return convertView;
    }


    public static class Holder {
           ImageView media;
           TextView caption;
    }
}

我是否在同一适配器中夸大了多个视图？谁能指出错误？