XCode 11. Closing debug window closes project.

Annoying issue.
If I close my designated debug window, the entire project closes.
Only applies to this project and no other.
Closing other windows work as normal.
Closing debug window last changes nothing if I reopen the other windows. Closing the debug window closes the project.
I have deleted the 'Derived Data' folder.
Is there some obscure setting causing this?

TIA Mark
XCode 11. Closing debug window closes project.
 
 
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