How to fix "No reply dictionary received from LeakAgent request" error in Xcode?
I had the same issue. It turned out the LeakAgent crashed somehow. So, I figured maybe launch instruments' leak tool should help. So I launched instruments, started the leaks tool and then closed the instruments window (not quit it). Returning back to XCode, the memory graph feature worked. Hope this will be of some help.
You might run into this issue if you are using Xcode 11.4.1 and trying to view memory graph hierarchy with a simulator with lower OS version (13.0 my case). Using the latest version of the simulator fixes the issue.
I took the hint that this problem was occurring on one device and not another that it could be related to iOS updates. Sure enough, all my pads were a couple minor version numbers behind. Once I flashed the newest iOS onto my iPads, the problem resolved itself.
Note: Restarts of macOS, iOS, as well as cleaning DerivedData had no effect.