I got this "Property "items" was accessed during render but is not defined on instance."

Hello, this is my project https://github.com/Erol-Mehmed/The-Learning-App .
I have a problem with this part <script>import { createApp } from "vue";createApp ({ data () { - Pastebin.com . I want to iterate through the array and display it, but I got the warning from the title of this post. I am totally new to Vue so I am at a loss. I set the Vue with this Quick Start | Vue.js.
Can you help me?