remove num

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    /**
     * @param head a ListNode
     * @param val an integer
     * @return a ListNode
     */
    public ListNode removeElements(ListNode head, int val) {
        // Write your code here
         // Write your code here
        while (head!=null){
            if (head.val == val) {
                head = head.next;
            }
            else break;
        }
        ListNode tmp = new ListNode(0);
        tmp.next = head;

        while (tmp.next != null && tmp.next.next!=null) {
                if (tmp.next.val == val) {
                    tmp.next = tmp.next.next;
                }
            tmp = tmp.next;
        }
        if (tmp!=null){

            if (tmp.val==val){
                tmp=tmp.next;
            }
        }
        if (tmp.next!=null){
            if (tmp.next.val==val){
                tmp.next=null;
            }
        }


        return head;

    }
}


-------------------
Runtime Error

Total Runtime: 3540 ms
97% test cases passed.
Input
5->6->6->null, 6
Expected
5->null
Error Message
Exception in thread "main" java.lang.NullPointerException at Solution.removeElements(Solution.java:42) at Main.main(Main.java:13)