Editor cannot load old file

Tested with https://python.microbit.org/v/beta, Load/Save "Or browse for a file": 
[mimic-19Sept20.zip](https://github.com/bbcmicrobit/PythonEditor/files/6130022/mimic-19Sept20.zip)

Loading file fails with error
 
![image](https://user-images.githubusercontent.com/989126/110940673-2c9b2a00-832f-11eb-9f25-b6ce052b2552.png)
