### Steps to reproduce

1. Login as a user with permissions to add relations

2. Go to any work package > Relations tab

3. Click on "+Relation"

4. Click on 'Existing child'

5. Do not populate the WP field and save


### What is the buggy behavior?

* 500 error is shown


### What is the expected behavior?

1. Form shows an Begin of the insertioninlineEnd of the insertion error text Begin of the insertion'Please select a work package'

2. Form cannot be savedEnd of the insertion Begin of the deletion (what error text: TBD)End of the deletion


### Screenshots and other files

<img class="op-uc-image op-uc-image_inline" src="/api/v3/attachments/272546/content">

### Environment information

**OpenProject version**

Core build

[2b7fe15cab91d5e40529a6d68393485a7bceb399](https://github.com/opf/openproject/commits/2b7fe15cab91d5e40529a6d68393485a7bceb399)

Product version

[201394be8a279687446002df993b8718ff96e38f](https://github.com/opf/saas-openproject/commits/201394be8a279687446002df993b8718ff96e38f)

Core version

OpenProject 15.3.0

**Browser**

* [ ] Chrome
* [x] Firefox 134.0.1
* [ ] Safari
* [ ] Mobile Safari
* [ ] Other (please specify)

**Operating System**

* [x] Windows 11Pro
* [ ] Mac OS X
* [ ] Mobile iOS
* [ ] Mobile Android
* [ ] Linux (please specify distro)
* [ ] Chrome OS
* [ ] Other (please specify)