I receive the same error as well.It seems like Xcode doesn't recognize the Experience.swift file from the Experience.rcproject file.There were times where I was somehow able to see the Experience.swift as I was toggling through the navigator pane.But I couldn't access it after a couple times, and gave me the same error.Apple, is this a version error?

Update: I just went ahead and built&run the app.It worked. I guess it needs to be built once for Xcode to recognize the .swift file.Hopefully it works with you.

Did you solve this issue in the end?I have the exactly the same issue with my Reality Composer. I tried editing on my iPad and I was able to add a text, but when I start editing it crashes everytime.I'm desperate to know the solution to it.Thank you very much.