Wow, I've been chasing this crash for some time and deselecting "Allow External Storage" made it go away. Thanks for the reply with the "fix".

Thanks for the update. Not really the answer we're looking for but I guess it clarifies their position.

I've similar concerns. The contexts from the first container will not be useable (at least in my testing). Short of tearing down the old core data stack, standing up the new one and traversing the view hierarchy to replace all existing managed objects and managed object contexts, there doesn't seem to be a way to toggle sync using NSPersistentCloudKitContainer. Not being able to pause or toggle sync seems to be a real shortcoming with NSPersistentCloudKitContainer. For kicks, I tried just disabling iCloud for the app Settings->AppleID->iCloud->App but that just deleted all the data on the device. I can live with the "eventual consistency" and not knowing sync progress, but not being able to toggle sync is a "show stopper".