sorting a Linked list manually in java. null pointer exception [duplicate]

I have a linked list with 5 objects each with a salary as follows:

999, 999, 9009, 700, 9000

I need to manually (without any built-in ava function) sort the linked list based on these salaries and here is my implementation of the sort method:

public static void sortBySalary() {

    tmp = head;
    int i = linkLength();
    System.out.println(i);
    head = null;
    Employee maxSalary = new Employee();
    int max = 0;
    Employee current = tmp;

    for (int b = 0; b < i; b++) {
        current = tmp;
        max = 0;
        for (int c = 0; c < i; c++) {
            if(head == null && max < current.salary) {
                maxSalary = current;
                max = current.salary;
            }

            else if( max < current.salary && current.salary < head.salary) {

                    maxSalary = current;
                    max = current.salary;


            }

            System.out.println(current.salary);
            current = current.next;
        }

        maxSalary.next = null;
        insertFirst(maxSalary);

    }


}

The error I get is after the second iteration of the first loop, the list seems to point at null when it reaches salary 9009. it runs through 900 then 900 then 9009 then it points at null.

I am also open to different approaches.