自定义适配器使用Inflator获取NullPointerException

Question

我有一个适配器，它从API中获取项目列表，然后将其放入列表视图中。这是我第一次使用适配器和列表视图，到目前为止，我正在努力奋斗！

为什么我收到此错误？我的背景错了还是什么？

package com.example.tvrplayer;

import java.io.File;
import java.util.ArrayList;
import java.util.HashMap;

import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

import android.app.Activity;
import android.content.Context;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseAdapter;
import android.widget.TextView;

public class HiddenChannelsListAdapter extends BaseAdapter {
    private Context mContext;
    private ArrayList mItems;
    private LayoutInflater mInflater;

    public HiddenChannelsListAdapter(Context ctx, ArrayList list) {
        mItems = list;
        mContext = ctx;
        mInflater = (LayoutInflater) mContext.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
    }

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

    @Override
    public HashMap getItem(int position) {
        return (HashMap) mItems.get(position);
    }

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

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        System.out.println("getView " + position + " " + convertView);
        if (convertView == null) {
            convertView = mInflater.inflate(R.layout.channel_item, parent, false);
        } else {
            ((TextView) convertView.findViewById(R.id.channel_text)).setText("Testing");
        }
        return convertView;
    }
    public static class ViewHolder {
        public TextView textView;
    }
}

Asyntask从服务器获取项目：

package com.example.tvrplayer;

import java.io.File;
import java.util.ArrayList;
import java.util.HashMap;

import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

import android.content.Context;
import android.os.AsyncTask;
import android.util.Log;
import android.view.View;
import android.widget.ListView;

class ChannelPair {
     public ListView lv;
     public ArrayList channelList;
     public Context ctx;
}

public class ChannelHandler extends AsyncTask<Object, Integer, ChannelPair> {

    @Override
    protected ChannelPair doInBackground(Object... params) {
        ChannelPair p = new ChannelPair();
        String apiurl = (String) params[0];
        String linkid = (String) params[1];
        String username = (String) params[2];
        String channelID = null;
        View vw = (View) params[3];

        ListView lv = (ListView) vw.findViewById(R.id.list);
        p.lv = lv;  

        JSONArray channels;
        ArrayList< HashMap < String, String > > channelList = new ArrayList < HashMap < String, String > > ();
        try {
            channels = Json.getJson(apiurl + "/rest/channel/"+ username +"/"+ linkid, "GET");
            Log.i("CHANNELS", channels.toString());
            for (int i=0; i < channels.length(); i++) { 
                JSONObject json_data = channels.getJSONObject(i);
                String name = json_data.getString("Name");
                String channelid = json_data.getString("ChannelID");
                HashMap<String, String> channelObject = new HashMap<String, String>();
//              if ( json_data.getString("ParentPath") == "" ) {
                channelObject.put("id", channelid);
                channelObject.put("name", name);
                channelList.add(channelObject);
//              }
            }
            Log.i("ROOT CHANNELS", channelList.toString());
            p.channelList = channelList;
        } catch (JSONException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        return p;
    }

    @Override
    protected void onPostExecute(final ChannelPair p) {
        HiddenChannelsListAdapter adapter = new HiddenChannelsListAdapter(p.ctx, p.channelList);
        p.lv.setAdapter(adapter);
    }
}

错误在这一行：

HiddenChannelsListAdapter.java:28
mInflater = (LayoutInflater) mContext.getSystemService(Context.LAYOUT_INFLATER_SERVICE);

完整的错误代码：

java.lang.NullPointerException
   at com.example.tvrplayer.HiddenChannelsListAdapter.<init>(HiddenChannelsListAdapter.java:28)
   at com.example.tvrplayer.ChannelHandler.onPostExecute(ChannelHandler.java:71)
   at com.example.tvrplayer.ChannelHandler.onPostExecute(ChannelHandler.java:1)
   at android.os.AsyncTask.finish(AsyncTask.java:602)
   at android.os.AsyncTask.access$600(AsyncTask.java:156)
   at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:615)
   at android.os.Handler.dispatchMessage(Handler.java:99)
   at android.os.Looper.loop(Looper.java:137)
   at android.app.ActivityThread.main(ActivityThread.java:4514)
   at java.lang.reflect.Method.invokeNative(Native Method)
   at java.lang.reflect.Method.invoke(Method.java:511)
   at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:790)
   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:557)
   at dalvik.system.NativeStart.main(Native Method)