Thanks for the explanation guys. The reason for the test was that I had similar code in my application. I create the ModelContainer in my App and set it on my DataManager, but it would be nil when trying to use it. It was tricky to find the issue.

Answering my own question. Found this blog, searching for answers https://blog.jacobstechtavern.com/p/swiftdata-outside-swiftui Thanks Jacob. The answer is simple, but maybe someone can explain. modelContext = container.mainContext works if it's modelContext = ModelContext(container) Done!