关于空指向错误异常，大家帮忙解决。

package org.ts.demo;class Emp {    private int empno;    private String ename;    private String job;    private double sal;    private double comm;    private Emp mgr;    private Dept dept;    public Emp(int empno, String ename, String job, double sal, double comm) {        this.empno = empno;        this.ename = ename;        this.job = job;        this.sal = sal;        this.comm = comm;    }    public void setMgr(Emp mgr) {        this.mgr = mgr;    }    public Emp getMgr() {        return this.mgr;    }    public void setDept(Dept dept) {        this.dept = dept;    }    public Dept getDept() {        return this.dept;    }    public String getEmpInfo() {        return "雇员信息：" + "\n\t|- 编号：" + this.empno + "\n\t|- 姓名：" + this.ename                + "\n\t|- 职务：" + this.job + "\n\t|- 工资：" + this.sal + "\n\t|- "                + SalGrade.getInstance(this.sal).getSalGradeInfo()  // 注释本行运行没有问题，但有这行却出现错误                + "\n\t|- 奖金：" + this.comm;    }}class Dept {    private int deptno;    private String dname;    private String loc;    private Emp[] emps;    public Dept(int deptno, String dname, String loc) {        this.deptno = deptno;        this.dname = dname;        this.loc = loc;    }    public void setEmps(Emp[] emps) {        this.emps = emps;    }    public Emp[] getEmps() {        return this.emps;    }    public String getDeptInfo() {        return "部门信息：" + "\n\t|- 编号：" + this.deptno + "\n\t|- 名称：" + this.dname                + "\n\t|- 位置：" + this.loc;    }}class SalGrade {    private int grade;    private double losal;    private double hisal;    private static final SalGrade salgrades[] = {            new SalGrade(1, 700.0, 1200.0), new SalGrade(2, 1201.0, 1400.0),            new SalGrade(3, 1401.0, 2000.0), new SalGrade(4, 2001.0, 3000.0),            new SalGrade(5, 3001.0, 9999.0) };    private SalGrade(int grade, double losal, double hisal) {        this.grade = grade;        this.losal = losal;        this.hisal = hisal;    }    public static SalGrade getInstance(double sal) {        for (int i = 0; i < salgrades.length; i++) {            if (sal > salgrades[i].losal && sal < salgrades[i].hisal) {                return salgrades[i];            }        }        return null;    }    public String getSalGradeInfo() {        return "工资等级：" + this.grade + "（" + this.losal + "~" + this.hisal + "）";    }}public class OODemo {    public static void main(String args[]) {        Dept dept = new Dept(10, "销售部", "上海");        Emp[] allEmp = { new Emp(1122, "张三", "干事", 1500.0, 50.0),                new Emp(2233, "李四", "销售", 2000.0, 100.0),                new Emp(3344, "王五", "经理", 3000.0, 100.0),                new Emp(4455, "赵六", "董事", 5000.0, 0.0) };        dept.setEmps(allEmp);        for (int i = 0; i < allEmp.length - 1; i++) {            allEmp[i].setMgr(allEmp[i + 1]);        }        for (int i = 0; i < allEmp.length; i++) {            allEmp[i].setDept(dept);        }        System.out.println(dept.getDeptInfo());        System.out.println("部门人数：" + dept.getEmps().length + "人");        System.out.println("-------------------------");        for (int i = 0; i < dept.getEmps().length; i++) {            Emp emp = dept.getEmps()[i];            System.out.println(emp.getEmpInfo());            if (emp.getMgr() != null) {                System.out.println(emp.getMgr().getEmpInfo());            }            System.out.println("-------------------------");        }    }}