Null Reference Exception for Class that doesn't inherit from Monobehaviour

I am trying to create a dialogue system following the Brackeys tutorial, and then edited to create a system for a conversation between 2 or more actors.
Now is throwing a reference exception for a class with no gameobject/ monobehaviour.

Here’s the the Dialogue Manager with the problem lines commented out under StartDialogue():

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;
using TMPro;

public class DialogueManager : MonoBehaviour
{

    [SerializeField]
    private AudioManager audioManager;
    public GameObject dialogueBox;
    public GameObject characterPortraitBox;
    private Queue<string> sentences;
    public TextMeshProUGUI nameText;
    public TextMeshProUGUI dialogueText;
   
    // Start is called before the first frame update
    void Start()
    {
        sentences = new Queue<string>();
        dialogueBox.SetActive(false);
    }
    private void Update()
    {
        AdvanceDialogue();
    }
    public void StartDialogue(Dialogue dialogue)
    {
        dialogueBox.SetActive(true);
        Time.timeScale = 0.01f;
        /**Fix Null Reference Exception on commented code bellow**/

        //nameText.text = dialogue.characterName;
        //characterPortraitBox.GetComponent<Image>().sprite = dialogue.charaterPortrait;
        //sentences.Clear();
        //foreach (string sentence in dialogue.sentences)
        //{
        //    sentences.Enqueue(sentence);

        //}
        DisplayNextSentence();

    }

    public void DisplayNextSentence()
    {
        if (sentences.Count == 0)
        {
            EndDialogue();
            return;
        }
        string sentence = sentences.Dequeue();
        StopAllCoroutines();
        StartCoroutine(TypeSentence(sentence));
    }
    public void EndDialogue()
    {
        Time.timeScale = 1f;
        Debug.Log("End of Conversation");
        dialogueBox.SetActive(false);
        //Steps to close out dialogue box.
    }
    IEnumerator TypeSentence(string sentence)
    {

        dialogueText.text = "";
        playCharacterSFX();
        foreach (char letter in sentence.ToCharArray())
        {
            dialogueText.text += letter;
            yield return null;
        }
    }
    private void playCharacterSFX()
    {
        if (nameText.text == "Daedalus")
        {
            audioManager.Play("Daedalus Typing");
        }
        if (nameText.text == "Wisp")
        {
            audioManager.Play("Wisp Typing");
        }
    }
    private void AdvanceDialogue()
    {
        if (Input.GetKeyUp(KeyCode.Return))
        {
            DisplayNextSentence();
            Debug.Log("Player pressed enter/return");
        }
    }
}

Here is the Class it is referencing:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

[System.Serializable]
public class Dialogue
{
    public Sprite charaterPortrait;
    public string characterName;
    [TextArea(3, 10)]
    public string[] sentences;

}

And here is the class that triggers the dialogue:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class DialogueTrigger : MonoBehaviour
{
    public Dialogue[] conversation;
    private Dialogue dialogue;
    void Start()
    {
        //TriggerDialogue();
    }
    public void TriggerDialogue()
    {
        FindObjectOfType<DialogueManager>().StartDialogue(dialogue);

    }
    public void OnMouseUp()
    {
        TriggerDialogue();
    }
}

Any advice?

Paste the actual error, it will have line numbers that will show the exact location of the problem.

Though at a glance, it doesn’t look like “dialogue” (line 8 of your last file) could possibly be anything besides null, because it’s private and its value never gets set to anything (I assume it should be set to one of the items in the “conversation” array). So when you call StartDialogue with that it’s going to send a null reference to that.

Thank you, for the prompt response! =)
Here is the error(s)
https://imgur.com/oQQXx7r

I’ll take another look another look at line 8.

There appears to be an issue with the link to the error, so here is the console:
NullReferenceException: Object reference not set to an instance of an object
DialogueManager.StartDialogue (Dialogue dialogue) (at Assets/Scripts/DialogueManager.cs:34)
DialogueTrigger.TriggerDialogue () (at Assets/Scripts/DialogueTrigger.cs:15)
DialogueTrigger.OnMouseUp () (at Assets/Scripts/DialogueTrigger.cs:20)
UnityEngine.SendMouseEvents:smile:oSendMouseEvents(Int32)

The image didn’t work. You can copy the error text straight out of the Console window.

Sorry, about that. I updated the post above before I saw this.