C# in Unity: Object reference not set to an instance of an object

Question

I'm fairly new to C# and haven't been able to work this one out. I'm trying to make a really simple text-based game where you select a text option and it presents the next prompt. I've got the bare bones of the script seemingly working, but when I click on the buttons I get the console error "Object reference not set to an instance of an object". Could anyone help me understand what this means? And how to fix it?

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

class StoryBlock {
    public string story;
    public string Option1Text;
    public string Option2Text;
    public string Option3Text;
    public StoryBlock Option1Block;
    public StoryBlock Option2Block;
    public StoryBlock Option3Block;

    public StoryBlock(string story, string Option1Text = "", string Option2Text = "", string Option3Text = "", 
    StoryBlock Option1Block = null, StoryBlock Option2Block = null, StoryBlock Option3Block = null)
    {
        this.story = story;
        this.Option1Text = Option1Text;
        this.Option2Text = Option2Text;
        this.Option3Text = Option3Text;
        this.Option1Block = Option1Block;
        this.Option2Block = Option2Block;
        this.Option3Block = Option3Block;
    }
}
public class GameManager : MonoBehaviour
{
    public TMP_Text StoryText;
    public Button Option1;
    public Button Option2;
    public Button Option3;

    StoryBlock currentBlock;

    static StoryBlock block1 = new StoryBlock("Hey are you there?", "Yeah I'm here", "No run", "Who's asking?", block2, block3, block4);

    static StoryBlock block2 = new StoryBlock("I never thought I'd see you again", "I wish you were dead", "Nice to see you too", "Who are you?");

    static StoryBlock block3 = new StoryBlock("I knew you'd say that", "You're a joke", "Again, who are you?", "Hey mom.");

    static StoryBlock block4 = new StoryBlock("I wish I'd never met you", "Cool", "Hell yeah", "Love you <3");

    void Start()
    {
        DisplayBlock(block1);
    }

    void DisplayBlock(StoryBlock block) {
        StoryText.text = block.story;
        Option1.GetComponentInChildren<TMP_Text>().text = block.Option1Text;  
        Option2.GetComponentInChildren<TMP_Text>().text = block.Option2Text;   
        Option3.GetComponentInChildren<TMP_Text>().text = block.Option3Text;  

        currentBlock = block;
    }

    public void Button1Clicked(){
        DisplayBlock(currentBlock.Option1Block);
    }
    public void Button2Clicked(){
        DisplayBlock(currentBlock.Option2Block);
    }
    public void Button3Clicked(){
        DisplayBlock(currentBlock.Option3Block);
    }
 }

A screenshot of the game

I've done some poking around to try to find a solution or better explaination for what this means but am having trouble.