Hello,
Yesterday I installed 10.14 beta 6 and now Xcode 10 beta 5 says :
Message from debugger: unable to attach.
On 10.14 beta 5 with Xcode 10 beta 5 this did not happen.
If I remove 'derived data' it works 1 time. 2nd time unable to attach.
Did also not happen on 10.14 beta 5.
Or, maybe I should wait for Xcode 10 beta 6.