Currently user can try to change type to milestone and as a result we show him a generic error if this Begin of the insertionwork packageEnd of the insertion Begin of the deletion workpackageEnd of the deletion has Begin of the insertionchildren. 

**Acceptance criteria:**

Change the generic error messageEnd of the insertion Begin of the deletion children\. Maybe we should disable milestone or show a custom popupEnd of the deletion to Begin of the insertion`Type cannot be a milestone becauseEnd of the insertion Begin of the deletion explain thatEnd of the deletion this Begin of the insertionwork package has children.`End of the insertion Begin of the deletion is not possible

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