I had the same problem.
In my case it was the following:
I had an ObservableObject in the App subclass that I was passing through environmentObject() to the content view.
I had forgotten to add a corresponding call to environmentObject() in ContentView_Previews.previews
Once I added that back in it stopped crashing.
struct ContentView_Previews: PreviewProvider {
static var previews: some View {
ContentView().environmentObject(Item())
}
}
I hope it helps :-)