Swift Fatal error: … Expected <some type> but found <some type>-CodePudding

My app uses CoreData. One of the entities is

final class ViewItem: NSManagedObject {
// …
}

During unit tests, I am trying to execute the following fetch request:

let viewItemFetchRequest = NSFetchRequest<ViewItem>(entityName: "ViewItem")

viewContext.performAndWait {
  do {
    let fetchedViewItems = try viewContext.fetch(viewItemsFetchRequest)
    let viewItems = Set(fetchedViewItems)
    // …
  }

Although the code executes correctly during normal operation, during unit tests it crashes.

When I set a breakpoint behind the try instruction, this breakpoint is reached, i.e. the fetch does not throw.
However I get a fatal error when I want to access fetchedViewItems, either by Set(fetchedViewItems) or with

(lldb) po fetchedViewItems
Fatal error: NSArray element failed to match the Swift Array Element type
Expected ViewItem but found ViewItem
2022-06-07 10:31:30.245019 0200 ShopEasy[42069:1864673] Fatal error: NSArray element failed to match the Swift Array Element type
Expected ViewItem but found ViewItem
1 value

Here is the relevant part of the stack frame:

What could be the reason and how could I fix the problem?

PS: I found some posts related to this error,

The code should only be present in your app target, and you should use @testable import ShopEasy to access app code in your unit tests.