The same thing is happening to me. I think it may be related to the fact I'm using the Xcode beta on Catalina rather than Big Sur.
mjfigueroa
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As a workaround, you can wrap the buttons with ToolbarItem(placement:):
.toolbar {
	#if os(iOS)
	ToolbarItem(placement: .navigationBarTrailing) {
		EditButton()
	}
	#endif
	ToolbarItem(placement: .navigationBarLeading) {
		Button("Add", action: makeSandwich)
	}
}
The second issue is most certainly a bug as well since I've been able to replicate it with a few machines running both Catalina and Big Sur. Not sure about the third issue since I'm not developing macOS apps at this time.
You can implement your error view in your root view with an if statement and show/hide the error view using either an environment object or a shared model/viewmodel property. This way you can control it directly from any view and/or from the model/viewmodel itself.