XCode 12.4 : An Internal Error Occurred Editing functionality may be limited

Since upgrading to XCode12.4 on two different MacBooks both running macOS 10.15.6 I have issues opening Storyboard / Xib files.

Just to be clear this doesn’t occur for a specific project – this happens for any project I open and for any storyboard / xib file within that project.

There is a long delay before the file opens and when it does I see.

1. The UI Components in blue outline rather than the details.

2. The Error Message: An Internal Error Occurred Editing functionality may be limited

From my log.txt

XCODE VERSION INFO:

Version = 12.4 (12D4e)
Plugin Version = 17701
Beta Version = 1
OS Version = 10.15.6 (19G2021)


ASSERTION:

Failed to update scene '01J-lp-oVM' with IBUIViewAutolayoutGuide (x1), UIView (x1), and UIViewController (x1).

Exception name: IBPlatformToolFailureException
Exception reason: The tool closed the connection

Application Specific Information:
** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Failed to create remote render context'

terminating with uncaught exception of type NSException
abort() called
CoreSimulator 732.18.6 - Device: iPhone Xʀ (9B855293-1F8D-4794-BF54-5193C46BB094) - Runtime: iOS 14.4 (18D46) - DeviceType: iPhone Xʀ

I’ve cleaned derived data / restarted my machine etc – all to no avail.

All I can do at this stage is reinstall XCode 12.4 unless anyone can make any suggestions?