Add null-checks to behandeling-van-agendapunt component (hotfix)

[elpoelma]

Overview

This PR adds several null/undefined checks to the behandeling-van-agendapunt component in order to prevent undefined errors.

connected issues and PRs:

None

How to test/reproduce

• Start the application
• Open up a meeting with agendapoints
• Remove an agendapoint
• The container is undefined error should no longer show up

Checks PR readiness

• UI: works on smaller screen sizes
• UI: feedback for any loading/error states
• Check cancel/go-back flows
• Check database state correct when deleting/updating (especially regarding relationships)
• changelog
• npm lint
• no new deprecations