NullReferenceException: Object reference not set to an instance of an object in Unity cause by coding

Question

    ansNum = Random.Range(0, ans.Count);
    ADisplay.AnsA = ans[ansNum];
    ans.RemoveAt(ansNum); 
    ansNum = Random.Range(0, ans.Count);
    ADisplay.AnsB = ans[ansNum];
    ans.RemoveAt(ansNum);
    ansNum = Random.Range(0, ans.Count);
    ADisplay.AnsC = ans[ansNum];
    ans.RemoveAt(ansNum);
    ansNum = Random.Range(0, ans.Count);
    ADisplay.AnsD = ans[ansNum];
    ans.RemoveAt(ansNum);

The debug log shown "NullReferenceException: Object reference not set to an instance of an object"

How to randomize data in list without repeat