"Result of 'MainView' initializer is unused" -- trying to filter NavigationView in SwiftUI

Question

I am trying to piece together a custom NavigationView in SwiftUI with Buttons for filtering. In this simple step I am just trying to change '.navigationTitle(title)' through filtering with three buttons ('header1', 'header2', 'header3'), however it is not working.

You can find a visual of this code here

ContentView:

struct ContentView: View {
 var body: some View {
     MainView(filter: .header1)
 }
}

MainView:

struct MainView: View {
    enum SourceType {
        case header1, header2, header3
    }
    
    @EnvironmentObject var sourceItems: SourceItems
    
    let filter: SourceType
    
    var body: some View {
        
        NavigationView {
            List {
                HeaderView()
                ForEach(0..<10) { sourceItem in
                    NavigationLink(destination: Text("Destination")) {
                        Text("Item: \(sourceItem)")
                    }
                }
            }
            
            .listStyle(.inset)
            .navigationTitle(title) // I am trying to make this change
            
        }
    }
    
    var title: String {
        switch filter {
        case .header1:
            return "Header 1"
        case .header2:
            return "Header 2"
        case .header3:
            return "Header 3"
        }
    }
}

struct MainView_Previews: PreviewProvider {
    static var previews: some View {
        MainView(filter: .header1)
            .environmentObject(SourceItems())
    }
}

HeaderView (This is where I get the warnings):

struct HeaderView: View {
    @StateObject var sourceItems = SourceItems()
    
    var body: some View {
        HStack {
            Button(action: {
                MainView(filter: .header1)
            }, label: {
                    Text("Header 1")
            })
            .buttonStyle(PlainButtonStyle())
            
            Spacer()
            
            Button(action: {
                MainView(filter: .header2)
            }, label: {
                    Text("Header 2")
            })
            .buttonStyle(PlainButtonStyle())
            
            Spacer()
            
            Button(action: {
                MainView(filter: .header3)
            }, label: {
                    Text("Header 3")
            })
            .buttonStyle(PlainButtonStyle())
        }
        .environmentObject(sourceItems)
    }
}

The code runs but the buttons don't filter.

Thanks for any help (I am new to SwiftUI)!