I was having the same (or similar) problem: crashing when more than one tab opens, resulting in the same error described in this discussion.
The problem does seem to be related to a bug when the toolbar method is called on the NavigationSplitView view. I also found that the problem does not occur when toolbar is called on a subview within NavigationSplitView.
In attempting to resolve this, I found that the problem also only occurs when the toolbar method is provided with a specific id parameter (i.e.: toolbar(id: "toolbar"){...}). If an id is not provided, I don't get an error.
Hopefully this helps narrow-down and isolate the specific cause of the problem.
Topic:
UI Frameworks
SubTopic:
SwiftUI
Tags: