TaskTYpe Once should check that schedule time is set

[sliverc]

Describe the bug

I added a new task in the Django Admin. For TaskType is assigned "Run once". I did not assign any value at schedule time, assigned all other values and pressed saved.

An exception was then raised:

TypeError at /admin/scheduler/task/1/change/

'<' not supported between instances of 'str' and 'datetime.datetime'

To Reproduce
Steps to reproduce the behavior:

1. Go to Django Admin
2. Click on 'Tasks Scheduler" -> Tasks.
3. Add a new task
4. assign all required values, choose TaskType "Run once" and leave the schedule time empty
5. Press "Save".

Expected behavior would be that when pressing save that I cannot save a task like this as it is invalid, and it shows me a message that I need to assign the schedule timed.

This I guess should be added to the Task.clean function see here https://github.com/django-commons/django-tasks-scheduler/blob/master/scheduler/models/task.py#L436

There it already checks for the other task types but not for "Run once"

[cunla]

Thanks for reporting this. Will commit fix shortly

[sliverc]

Great. Thanks for addressing it so quickly and your effort working on this project. Very much appreciated.