Caused by: java.lang.NullPointerException: Attempt to invoke virtual method ‘java.lang.StringBuilder java.lang.StringBuilder.append(java.lang.String)‘ on a null object reference

private StringBuilder newStringBuilder() {
        // TODO Auto-generated method stub
        return null;
    }

package com.fmd.studentmanager;

import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;

import android.app.Activity;
import android.os.Bundle;
import android.text.TextUtils;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.EditText;
import android.widget.RadioGroup;
import android.widget.Toast;

public class MainActivity extends Activity {

    private EditText et_name;
    private EditText et_number;
    private RadioGroup rg_sex;
    
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        et_name = findViewById(R.id.et_name);
        et_number = findViewById(R.id.et_number);
        rg_sex = findViewById(R.id.rg_sex);
    }
    
    public void save(View view){
        String name = et_name.getText().toString().trim();
        String number = et_number.getText().toString().trim();
        if(TextUtils.isEmpty(name)||TextUtils.isEmpty(number)) {
            Toast.makeText(this, "学生的姓名或学号不能为空", 0).show();
            return;
        }
        StringBuilder sb = new StringBuilder();
        int id = rg_sex.getCheckedRadioButtonId();
        String sex = "male";
        if(id == R.id.rb_male) {
            //男性
            sex = "male";
        }else {
            //女性
            sex = "female";
        }
        //数据保存到文件
        sb.append("<?xml version=\"1.0\" encoding=\"utf-8\"?>");
        sb.append("<student>");
        sb.append("<name>");
        sb.append(name);
        sb.append("</name>");
        sb.append("<number>");
        sb.append(number);
        sb.append("</number>");
        sb.append("<sex>");
        sb.append(sex);
        sb.append("</sex>");
        sb.append("</student>");
        try {
            File file = new File(getFilesDir(),name+".xml");
            FileOutputStream fos = new FileOutputStream(file);
            fos.write(sb.toString().getBytes());
            fos.close();
            Toast.makeText(this, "数据保存成功", 0).show();
        } catch (Exception e) {
            Toast.makeText(this, "数据保存异常", 0).show();
            e.printStackTrace();
        }
    }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }
}