SwiftUI - Add a Navigation Bar Button on Condition

As .navigationBarItems will be deprecated in a future version of iOS it's better to use .toolbar(_:) View Modifier.

.toolbar {
    ToolbarItemGroup(placement: .navigationBarTrailing) {
        if tabIndex == 2 {
            // show button
        }
    }
}

Here is possible approach

.navigationBarItems(trailing: self.tabIndex == 1 ? 
    AnyView(self.trailingButton) : AnyView(EmptyView()))

somewhere below body

var trailingButton: some View {
  Button(action: {print("Button was tapped")}) {
   Image(systemName: "plus")
    .resizable().frame(width: 20, height: 20)
  }
}