fix(editor): creating custom categories

[st3iny]

Fix #5993

https://github.com/nextcloud/calendar/assets/1479486/20158423-ac96-4c3b-9419-df74c32fb517

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 23.51%. Comparing base (397c78e) to head (403ed95). Report is 13 commits behind head on main.

Files	Patch %	Lines
...nents/Editor/Properties/PropertySelectMultiple.vue	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #5994 +/- ## ========================================= Coverage 23.51% 23.51% Complexity 455 455 ========================================= Files 246 246 Lines 11722 11722 Branches 2156 2156 ========================================= Hits 2756 2756 - Misses 8650 8651 +1 + Partials 316 315 -1 ``` | [Flag](https://app.codecov.io/gh/nextcloud/calendar/pull/5994/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nextcloud) | Coverage Δ | | |---|---|---| | [javascript](https://app.codecov.io/gh/nextcloud/calendar/pull/5994/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nextcloud) | `15.15% <0.00%> (ø)` | | | [php](https://app.codecov.io/gh/nextcloud/calendar/pull/5994/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nextcloud) | `58.46% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nextcloud#carryforward-flags-in-the-pull-request-comment) to find out more.

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[st3iny]

/backport to stable4.7