Inconsistencies in how the app reacts to invalid data in the json file

[tyanhan]

Changing duration to 0 in the json file causes app to be unable to start up

Changing time to invalid format like HH:mm:ss allows the app to start, but starts with no data

Expected:
Changing duration to 0 should still allow app to start, but with no data

To reproduce:

1. Add a lesson
2. Goto the json file and edit the duration to 0hours and minutes.
3. Try starting the app, the app should not be able to start
4. Change the duration back to non zero in the json file
5. Change the time to HH:mm:ss format in the json file
6. Start the app again. This time it runs but starts with no data.