In case anyone else gets stuck following the video tutorial to create the iDine App from SwiftUI by Example in Xcode 12, here is a fix.

Don't create an App Delegate or Scene Delegate. At around 50 minutes in the video instead of editing Scene Delegate change iDineApp.swift to this:

import SwiftUI

@main
struct iDineApp: App {

    var order = Order()

    var body: some Scene {
        WindowGroup {
            AppView().environmentObject(order)
        }
    }
}