I had the same thing. The latest Xcode update breaks something in the api debugger. (I suspect this is just a change in the debugserver command flags.)
Currently, the best solution I know is to uninstall the Xcode or Xcode command-line tools and install the 9.2 command-line tools for MacOS High Sierra or for MacOs Sierra .
Install and do not update Xcode until it is fixed in either Xcode or Delve.
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