[Vuejs]-DOMException occurred in Vue: Failed to execute 'insertBefore' on 'Node': The node before which the new node is to be inserted is not a child of this

Your issue stemmed from package incompatibilities.

You were using vue@3 + @vue/composition-api@1. Apart from those, you had quite a few linting and typescript errors and some missing dependencies.

To save time, I created a new Vue3 project (with typescript, vuex & router) and copy/pasted your components & folders into it. I added the missing dependencies and fixed a bunch of linting and ts errors.

When serving, I always get the sidebar when navigating to /lecture/lecture_home_page:

PR-ed here. Read the notes.

Take some time to read (and fix) all the Vue warnings in console. Some of them are important.