Error with cam json on Import Data

Getting an error opening the cam json file from blendartrack app. Using Windows 10, Blender 3.6. Don't seem to get errors with the json files intrinsic, screen or anchor when clicking Import Data for them. 

Added image of the error:

![blendartrack-error](https://github.com/cgtinker/blendartrack/assets/5981075/56aa64c2-a111-4185-94e9-689369b75a02)
