Error saving notebook

[kpeeters]

I am building a Jupyter kernel for the Cadabra computer algebra system (https://cadabra.science), using the Xeus library to do the communication. Most works well, but if I make even the simplest notebook and then try to save it, Jupyter comes back with a dialogue

The save operation succeeded, but the notebook does not appear to be valid. The validation error was:

Notebook validation failed: None is not of type 'object':
null

The same error pops up when I exit Jupyter, restart, and then try to load the saved notebook again.

A simple notebook is listed below. Removing the single empty 'code' cell with 'execution_count: null' has no effect. Any idea what causes this error, and more importantly, how I can avoid it?

{
 "cells": [
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/markdown": [
       "${}A_{m n}$"
      ]
     },
     "execution_count": 1,
     "metadata": null,
     "output_type": "execute_result"
    }
   ],
   "source": [
    "ex:=A_{m n};"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": []
  }
 ],
 "metadata": {
  "kernelspec": {
   "display_name": "Cadabra",
   "language": "python",
   "name": "cadabra"
  },
  "language_info": {
   "file_extension": ".cdb",
   "mimetype": "text/cadabra",
   "name": "cadabra",
   "version": "2.0.0"
  }
 },
 "nbformat": 4,
 "nbformat_minor": 2
}