我正试图通过单击 ImageButton 在我的警报活动中，计数器位于server.java类（这是一个tcp服务器）中，但是当我试图通过在我的alert.java中设置来更改值时：

Server server;
    refreshing.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            server.count = 0;
            SharedPreferences prefs = getSharedPreferences("MY_DATA", MODE_PRIVATE);
            SharedPreferences.Editor editor = prefs.edit();
            editor.clear();
            editor.apply();
            Intent intent = getIntent();
            overridePendingTransition(0, 0);
            intent.addFlags(Intent.FLAG_ACTIVITY_NO_ANIMATION);
            finish();
            overridePendingTransition(0, 0);
            startActivity(intent);
        }
    });

错误导致崩溃： Java.lang.NullPointerException: Attempt to write to field 'int com.example.sguidetti.selfmanegment.Server.count' on a null object reference 当我在主活动中使用相同的操作通过单击另一个来重置相同的计数器时 ImageButton 这就行了。
（附言：我是android新手，所以我不懂很多东西）
这里编辑的是server.java

public class Server {
DataBaseHandler myDB;
allert Allert;
MainActivity activity;
RecyclerViewAdapter adapterView;
Adapter adapter;
ServerSocket serverSocket;
public static int count=0;
String letto = "";
private SharedPreferences prefs;
static final int socketServerPORT = 8080;

public Server(MainActivity activity) {
    this.activity = activity;
    Thread socketServerThread = new Thread(new SocketServerThread());
    socketServerThread.start();
}

public int getPort() {
    return socketServerPORT;
}

public void onDestroy() {
    if (serverSocket != null) try {
        serverSocket.close();
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
}

private class SocketServerThread extends Thread {
    Vibrator vibrator;
    String date,ora;
    long[] pattern = {0, 1000, 500, 1000, 500, 1000};

    int lun;

    @Override
    public void run() {
        InputStream leggi;
        try {

            serverSocket = new ServerSocket(socketServerPORT);

            while (true) {
                myDB = DataBaseHandler.getInstance(activity);

                Socket socket = serverSocket.accept();
                leggi = socket.getInputStream();
                byte[] data = new byte[1000];
                lun = leggi.read(data, 0, data.length);
                letto = new String(data, "UTF-8");
                count++;
                MediaPlayer mPlay = MediaPlayer.create(activity, R.raw.gabsuono);
                mPlay.start();

                vibrator = (Vibrator) activity.getSystemService(VIBRATOR_SERVICE);
                vibrator.vibrate(pattern, -1);

                date = new SimpleDateFormat("dd-MM-yyyy").format(new Date());
                ora = new SimpleDateFormat("HH:mm:ss").format(new Date());
                myDB.insertDataServer(date, ora, letto);

                //adapterView.notifyDataSetChanged();

                activity.runOnUiThread(new Runnable() {
                    @Override
                    public void run() {
                        prefs = activity.getSharedPreferences("MY_DATA", MODE_PRIVATE);
                        SharedPreferences.Editor edit = prefs.edit();
                        edit.putInt("counter", count);
                        edit.commit();
                        activity.msg.setText(String.valueOf(count));
                        activity.msg.setVisibility(View.VISIBLE);

                    }
                });
                leggi.close();

            }
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();

        }

    }

}

public void Parti() {
    prefs = activity.getSharedPreferences("MY_DATA", MODE_PRIVATE);
    count = prefs.getInt("counter", count);
    activity.msg.setText("" + count);
    if (count == 0)
        activity.msg.setVisibility(View.INVISIBLE);
    else
        activity.msg.setVisibility(View.VISIBLE);

}

}