Unity game crashing when dialogue with NPC initiated [duplicate]

I have 3 NPCs in my "Game". When the player walks up to any of them, they're supposed to have their own dialogue; I've tested this, and they do.

I tried to create a display that shows this dialogue text using a TextMeshProUGUI attached to a blank GameObject called dialogDisplay (tagged DialogDisplay). There is one DialogDisplay object in the scene that I'm trying to get each of my NPC's to use. I tried to make the display object inactive in the NPC Start() function, and enable it when it's time to display the NPC's next sentence.

When I run my game, I get an error on line 21, where SetActive() is called: "Object Reference Not Set to an Instance of an Object".

Here's the NPC script (DisplayNextSentence() is at the bottom, where dialogDisplay is used):

public class NPC : Character {
private bool charInRange;
public Dialogue dialogue;
public bool talkedTo = false;
private Queue<string> sentences;
public TextMeshProUGUI textPro;
public GameObject dialogDisplay;
private bool isTalking;

// Use this for initialization
void Start () {
    charInRange = false;
    textPro = FindObjectOfType<TextMeshProUGUI>();
    dialogDisplay = GameObject.FindGameObjectWithTag("DialogDisplay");
    dialogDisplay.SetActive(false);
}

// Update is called once per frame
void Update () {
    //if player is in range and presses space, triggers NPC dialogue
    if (charInRange && Input.GetKeyDown(KeyCode.Space))
    {
        TriggerDialogue();
    }
}

//if Player gameObject is in NPC collider, player is in range
void OnTriggerEnter2D(Collider2D other)
{
    if(other.gameObject.tag == "Player")
    {
        charInRange = true;
    }
}

//if player exits NPC collider, player is not in range
void OnTriggerExit2D(Collider2D other)
{
    if (other.gameObject.tag == "Player")
    {
        charInRange = false;
    }
}

//if NPC has been talked to before, displays next sentence; if not, loads dialogue and displays first sentence
private void TriggerDialogue()
{
    if (!talkedTo)
    {
        talkedTo = true;
        StartDialogue(dialogue);
    }
    else
    {
        DisplayNextSentence();
    }
}

//loads a queue with lines from Dialogue and displays first sentence
public void StartDialogue(Dialogue dialogue)
{
    sentences = new Queue<string>();

    foreach (string sentence in dialogue.sentences)
    {
        sentences.Enqueue(sentence);
    }

    DisplayNextSentence();
}

//displays next sentence in the queue
public void DisplayNextSentence()
{
    string sentence;
    bool done = false;

    //if last sentence in the queue, display it again
    if (sentences.Count == 1)
    {
        sentence = sentences.Peek();
        textPro.text = sentence;
        Debug.Log(sentence);
        return;
    }

    sentence = sentences.Dequeue();
    textPro.text = sentence;
    dialogDisplay.SetActive(true);
    while (!done) {
        Debug.Log("In WHile Looop");
        if (Input.GetKeyDown(KeyCode.A))
        {
            done = true;
        }
    }
    dialogDisplay.SetActive(false);
}
}

Grateful for any insight or tips you can provide. Happy to provide further information if it helps. Thank you!