Fix child schedule failing to load after del parent

What does this MR do and why?

Resolves #404 (closed)

• Fixes child schedules failing to load after deleting their parent.
• Child schedules were correctly returning the parent as none, but we were not handling that on the getter for the frontend.

Screenshots or screen recordings

How to set up and validate locally

1. Create two schedules. One parent and one child
2. Delete the parent
3. Click the edit button the child

MR acceptance checklist

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• Have you checked this doesn't interfere/conflict/duplicate someone elses work?
• Have you fully tested your changes?
• Have you resolved any lint issues?
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