I can recreate the following error.
- Use a boolean hub variable in ML to disable on and/or off
- Rename that hub variable
- Go back to that ML instance and you will not be able to open the child app
- You will get "Error: Cannot get property 'value' on null object
Workaround is to rename the hub variable back to its original form, remove it from the ML instance, change the hub var name, and then add it back to ML.
Hope this helps somebody. Not a biggie.