"Null check operator used on a null value" at ListView Builder

I have this seperate page that I navigate to it and I am testing my Question widget with a ListView.builder(). The error occurs when I navigate to the page, at ListView.builder(). I am new to ListView/GridView builders so I followed some tutorials about it. I can't figure out the issue here.

import 'package:flutter/material.dart';

import 'consonants.dart';
import 'functions.dart';
import 'question_bank.dart';

int sum = 0;

class HTP extends StatefulWidget {
  HTP({ Key? key }) : super(key: key);

  @override
  State<HTP> createState() => HTPState();
}

class HTPState extends State<HTP> {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      title: title,
      home: Scaffold(
        backgroundColor: Color(0xFF88FFEF),
        appBar: AppBar(
          backgroundColor: theme_color,
          title: const Text(title,
            style: TextStyle(
              fontSize: 25,
              fontWeight: FontWeight.w400,
              color: Colors.white
            )
          )
        ),
        body: ListView(
          padding: const EdgeInsets.all(5),
          children: [
            Text('TITLE',
              style: TextStyle(
                fontSize: 28, fontWeight: FontWeight.w700)
            ),
            Question(
              'A',
              '1. ...\n2. ...',
              ['a', 'b', 'c'],
              ['etc 1', 'etc 2', 'etc 3']
            )
          ]
        )
      )
    );
  }

  Widget Question(String group_name, String conditions, List<String> subgroups, List<String> options) {
    return Column(
      children: [
        Text('GROUP $group_name',
          style: TextStyle(
            fontWeight: FontWeight.w800,
            fontSize: 18,
            color: theme_color
          )
        ),
        Align(
          alignment: Alignment.centerLeft,
          child: Text(conditions,
            style: TextStyle(fontSize: 16)
          )
        ),
        Align(
          alignment: Alignment.centerLeft,
          child: ListView.builder(
            itemCount: question_points.length,
            itemBuilder: (context, index) {
              return ListTile(
                title: Text(question_points[index].toString())
              );
            }
          )
        )
      ],
    );
  }
}

I am going to display the arguments passed to the widget without using a for loop.