### Steps to reproduce

1. Create a WP of a type which is not a Milestone

2. Make sure, that the WP has duration larger than 1 Begin of the insertiondayEnd of the insertion

3. Make sure, that the WP is automatically scheduled

1. Begin of the deletion add a predecessor (with dates set)

2. End of the deletion and add a child (with dates Begin of the insertionset with a duratino larger than 1 day)End of the insertion Begin of the deletion set)End of the deletion

4. Try to change the type of the WP to a Milestone type


### What is the buggy behavior?

* There is an error, because Begin of the insertiona work package with children cannot be a milestone, and it also tries to setEnd of the insertion the Begin of the insertiondates to the children's dates which generates additional errors: theEnd of the insertion finish date can't be changed (as it is automatically scheduled) but a Milestone needs to have the finish date the same as the start date. 


<img class="image_resized op-uc-image op-uc-image_inline" style="width:582px;" src="/api/v3/attachments/389769/content"> Begin of the deletion

End of the deletion <img class="op-uc-image op-uc-image_inline" src="/api/v3/attachments/389773/content">

### What is the expected behavior?

* Begin of the insertionOnlyEnd of the insertion Begin of the deletion ChangingEnd of the deletion the Begin of the insertionerror &quot;Type cannotEnd of the insertion Begin of the deletion type shouldEnd of the deletion be Begin of the insertiona milestone because this work package has children&quot;End of the insertion Begin of the deletion possible and the datesEnd of the deletion should Begin of the insertionbe displayedEnd of the insertion Begin of the deletion adaptEnd of the deletion


Begin of the deletion ###

End of the deletion ### Environment information

**OpenProject version**

_15.4_

**Browser**

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

**Operating System**

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

**Language**

German