Custom topNode that uses addExtensions to include defaultExtensions throws error

[tobiasfuhlroth]

Description
When using addExtensions on a custom topNode to include mandatory extensions (like Text), tiptap throws an Error saying: RangeError: Schema is missing its top node type ('doc')

Steps to reproduce the bug
Steps to reproduce the behavior:

1. Go to the CodeSandbox
2. Reload App
3. See error message

CodeSandbox
I created a CodeSandbox to help you debug the issue:
https://codesandbox.io/s/tiptap-issue-template-forked-k5xxp?file=/src/components/Tiptap.vue

Expected behavior
I should just work 😄

Additional context
If you remove addExtensions from Letter and add Paragraph and Text in the editors extensions directly it works.