System.out.println("欢迎你！" + student.name+"。请注意每人限选5门课程");  for(int i=0;i<5;i++){   System.out.println("请输入课程ID");   scan3 = new Scanner(System.in);   String input3 = scan3.nextLine();   for(Course courses:coursesToSelect){    if(courses.id.equals(input3)){     student.courses.add(courses);     //  运行之后显示这一行出现了空指针异常，怎么解决？}   }  }