# Load a JsonObject associated to `key` from store.
	#
	# Ensure `has_data(key)`
	private fun load_json(key: String): nullable Serializable do
		assert has_key(key)
		var path = "{store_dir}/{key}.json".simplify_path
		var file = new FileReader.open(path)
		var text = file.read_all
		file.close
		return text.parse_json
	end