My Assistant Editor always opens header files from the Xcode 11 directory.
For example, I tried opening the ContentView.swift file associated with my view Object. Instead the Assistant Editor opened a UIViewController.h file, which isn't even in my folder. As far as I understand it, it's a directory issue and I tried to set the file manually but nothing happens. It's like Xcode doesn't let me use anything else than the .h files.
I tested with XCode 11.2ß2, selecting a controller in IB and then call Assistant via the menu.
It opens the right file.
But there have been a few threads on this.
What you may have to do is select the "Navigate to related items" at the top left of the canvas window (icon shows 2 small stacked rectangles) and select top level.
Or, in the code window displayed next to the IB Canvas, click on Automatic (in the top bar) and select Top Level Objects.