Unhandled error event and cannot read properties of null nodemon app crashed

Question

[nodemon] starting `node app.js`
Server started on port 3000
node:events:505
      throw er; // Unhandled 'error' event
      ^

TypeError: Cannot read properties of null (reading 'items')
    at C:\Users\Debosmita\Desktop\Web Dev\todolist-v2-starting-files\app.js:97:19
    at C:\Users\Debosmita\Desktop\Web Dev\todolist-v2-starting-files\node_modules\mongoose\lib\model.js:5010:18
    at processTicksAndRejections (node:internal/process/task_queues:78:11)
Emitted 'error' event on Function instance at:
    at C:\Users\Debosmita\Desktop\Web Dev\todolist-v2-starting-files\node_modules\mongoose\lib\model.js:5012:15
    at processTicksAndRejections (node:internal/process/task_queues:78:11)
[nodemon] app crashed - waiting for file changes before starting..

This is the app.js file

//jshint esversion:6

const express = require("express");
const bodyParser = require("body-parser");
const mongoose = require("mongoose");

const app = express();

app.set("view engine", "ejs");

app.use(bodyParser.urlencoded({ extended: true }));
app.use(express.static("public"));

mongoose.connect("mongodb://localhost:27017/todolistDB", {
  useNewUrlParser: true,
});

const itemsSchema = {
  name: String,
};

const Item = mongoose.model("Item", itemsSchema);

const item1 = new Item({
  name: "Welcome to your todolist!",
});

const item2 = new Item({
  name: "Hit the + button to add a new item.",
});

const item3 = new Item({
  name: "<-- Hit this to delete an item.",
});

const defaultItems = [item1, item2, item3];

const listSchema = {
  name: String,
  items: [itemsSchema],
};
const List = mongoose.model("List", listSchema);

app.get("/", function (req, res) {
  Item.find({}, function (err, foundItems) {
    if (foundItems.length === 0) {
      Item.insertMany(defaultItems, function (err) {
        if (err) {
          console.log(err);
        } else {
          console.log("Successfully saved default items to DB.");
        }
      });
      res.redirect("/");
    } else {
      res.render("list", { listTitle: "Today", newListItems: foundItems });
    }
  });
});

app.get("/:customListName", function (req, res) {
  const customListName = req.params.customListName;
  List.findOne({ name: customListName }, function (err, foundList) {
    if (!err) {
      if (!foundList) {
        //Create a new list
        const list = new List({
          name: customListName,
          items: defaultItems
        });
        list.save();
        res.redirect("/" + customListName);
      } else {
        //Show an existing list
        res.render("list", {
          listTitle: foundList.name,
          newListItems: foundList.items,
        });
      }
    }
  });
});

app.post("/", function (req, res) {
  const itemName = req.body.newItem;
  const listName = req.body.list;
  const item = new Item({
    name: itemName,
  });

  if (listName === "Today") {
    item.save();
    res.redirect("/");
  } else {
    List.findOne({ name: listName }, function (err, foundList) {
      if (!err) {
        foundList.items.push(item);
        foundList.save();
        res.redirect("/" + listName);
      } else {
        console.log(err);
      }
    });
  }
});

app.post("/delete", function (req, res) {
  let checkedItemId = req.body.checkbox;
  if (!mongoose.Types.ObjectId.isValid(checkedItemId)) {
    checkedItemId = checkedItemId.replace(/\s/g, "");
  }
  Item.findByIdAndRemove(checkedItemId, function (err) {
    if (!err) {
      console.log("Successfully deleted checked item.");
      res.redirect("/");
    }
  });
});

app.get("/about", function (req, res) {
  res.render("about");
});

app.listen(3000, function () {
  console.log("Server started on port 3000");
});