### Steps to reproduce

1. Login as admin

2. Go to the Administration/Work Packages/Types/Form

3. Remove Assignee field from the Form of type Milestone

4. Create Work Package view including Assignee column in a project that has Milestone.


### What is the buggy behavior?

* The Work Package view shows the Assignee column of the Milestone as an active field, and makes it possible to select a user as an Assignee

* The Overview page of the Work Package does not show (correctly) the Assignee field since it was removed from the form.


### What is the expected behavior?

1. The Work Package view should not show the Assignee column of the Milestone as an active field, and it should not be possible to select a user as an Assignee Begin of the insertion

2. not applicable Assignee field would be implemented in the Excel export function (i.e. an automatic "n/a" designation is inserted in the relevant Excel cells).End of the insertion


### Screenshots and other files

**GIF uploaded.**

### Environment information

<br>

**OpenProject installation type**

* **Docker All-in-one container**

<br>


**OpenProject version**

**v15.1.0**

**Browser**

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

**Operating System**

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