自定义gridview的getview方法中的null对象引用错误

Question

我在holder.text.setText（项目）时遇到错误。请给我一些建议。

package com.example.dipen.custom_listview;

import android.content.Context;
import android.content.res.Resources;
import android.graphics.Color;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseAdapter;
import android.widget.GridView;
import android.widget.ImageView;
import android.widget.ListView;
import android.widget.TextView;

import java.util.ArrayList;


public class MainActivity extends ActionBarActivity {

    GridView list;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        `setContentView`(R.layout.activity_main);
       list = (GridView) findViewById(R.id.gridView);
        list.setAdapter(new customadapter(this));
    }


    class singlerow{

        String name;
        int image;
        singlerow(String name,int image){
            this.name=name;
            this.image=image;
        }
    }

    class customadapter extends BaseAdapter{

        ArrayList<singlerow> list;
        Context context;
        customadapter(Context c){
            context=c;
            list=new ArrayList<singlerow>();
            Resources res=c.getResources();
            String[] str=res.getStringArray(R.array.name);
            int[] imag={R.drawable.autocomplete_java,R.drawable.autocomplete_output,R.drawable.autocomplete_xml};
            for(int i=0;i<3;i++){

               list.add(new singlerow(str[i],imag[i]));

            }
        }

        @Override
        public int getCount() {

            return list.size();
        }

        @Override
        public Object getItem(int i) {

            return list.get(i);
        }

        @Override
        public long getItemId(int i) {

            return i;
        }

         class  viewholder{
            TextView text;
            ImageView image;
            viewholder(View view){
                 text= (TextView) view.findViewById(R.id.textView);
                image=(ImageView)view.findViewById(R.id.imageView);
            }
        }

        @Override
        public View getView(int i, View view, ViewGroup viewGroup) {
            View row=view;
            viewholder holder;
            if(row == null) {
                LayoutInflater inflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
                row = inflater.inflate(R.layout.single_row, viewGroup, false);
                holder=new viewholder(row);
            }
            else{
                holder=(viewholder)row.getTag();

            }
            TextView text= (TextView) row.findViewById(R.id.textView);
            ImageView image=(ImageView)row.findViewById(R.id.imageView);
            singlerow temp=list.get(i);





  holder.text.setText(temp.name);
            holder.text.setTag(temp);
           holder.image.setImageResource(temp.image);
            holder.image.setTag(temp);
            holder.image.setTag(temp);
            if (i % 2 == 1) {
                row.setBackgroundColor(Color.YELLOW);
            } else {
                row.setBackgroundColor(Color.CYAN);
            }
            return row;
        }
    }
}