Time profiler in instruments is not working
I was hitting the same issue for a week.
Since today, I change the Xcode Version back to Xcode 9.2 - and the problem was resolved.
It seems like this is a bug of Xcode 9.3, I just filed an issue to Apple yesterday. No reply has been received.
I had two versions of Xcode installed, and was experiencing this when using the Instruments associated with the oldest version of Xcode (9.4.1).
I solved the issue by using the Instruments associated with the newest version of Xcode I had installed (10.0)