I've been having the same issue, with all newly exported Xcode projects not loading and causing Xcode to crash in both 12 and 12.0.1. But, I was playing around with it just now and found a workaround that seems to work, for me at least.
I exported the Construct 3 project as an Xcode project, then opened the Xcode workspace file (e.g. "gamename.xcworkspace") in Xcode 11.7. That opened fine. I then closed Xcode and opened the workspace file again but this time in Xcode 12.0.1. This time it loads and works fine.
If I make changes to my Construct project then I have to go through these steps again.
I'd hazard a guess that Xcode 11.7 makes a change of some sort to the project file when it's opened or closed, because if I attempt to open it in 12.0.1 before going through the steps above I just get Xcode crashing before any windows appear.
Hope this works for others who are having this problem.
Thanks for sharing your solution, it worked perfect for me