I know that this is an old subject, but I found the issue with xcode 12.3 and was related to an error while doing the CopyPlist of the main.Storyboard during compilation.
Actually, changing the build settings to "Legacy Build Setting" worked, but it is deprecated, so I discarded it because is a short term solution.
Check this:
With that setting, worked for me. Before I had "Copy plist".