Looks fine in light mode, text unreadable in dark mode.

Describe the bug
When exporting annotation with Flashcard, the JSON line is undefined

    },
    undefined,
    {

To Reproduce
Steps to reproduce the behavior:

1. Annotate a line of text
2. Add a Flashcard with data on back and front
3. Export annotation as JSON
4. Search "undefined" you should get to that badly exported entry

Expected behavior

Fla

Is your feature request related to a problem? Please describe.
When the background pdf could not be found, please mention full path of the pdf that was connected with the xopp file.

Describe the solution you'd like
Mention full path of the pdf that was connected with the xopp file.

Describe alternatives you've considered

Additional context

Describe the solution you'd like
The private API should provide a way to delete all revisions of a note

Additional context
This implements an aspect of documentation merged in hedgedoc/hedgedoc#1033