Infinite loading upon clearing text in a date picker

[ziqing26]

Environment
Live server at V8.6.0

Steps to reproduce

1. Choose a feedback session to edit (go to Edit Feedback Session Page)
2. Delete the date for any date picker (for example, Submission opening time or Make session visible)
3. Click 'Save Changes" button.
   

Expected behaviour
Report an error for invalid date time and stop loading to save data.

Actual behaviour
No error is reported and the loading sign is shown to prevent further editing (does not stop trying to save).

Additional info
Might need to catch this error before it attempts to save the data in the front end.

[FergusMok]

Sounds good, I'll try to fix this issue