Implementation: fix checking the username so it won't generate undefined behavior during ui rendering

Implementation/Peered/ContentView.swift

@@ -12,22 +12,24 @@ extension EnvironmentValues {
 }
 
 struct ContentView: View {
-  @AppStorage("peered_username") private var username: String = "fallback_user"
-  @State private var notes = [Note]()
+	@AppStorage("peered_username") private var username: String = "fallback_user"
+	@State private var notes = [Note]()
 	@State private var notesClient: NoteEditingSessionClient?
 	@State private var ownPeer: OwnPeer?
 	
-  var body: some View {
+	var isUsernameValid: Bool {
+		username != "fallback_user" && !username.isEmpty
+	}
+	
+	var body: some View {
 		NavigationStack {
 			List {
 				Section("Your notes") {
 					ForEach(notes) { note in
 						NavigationLink(note.name) {
-							let peer = ownPeer ?? .init(peer: .init(displayName: username))
-							if ownPeer == nil {
-								ownPeer = peer
+							if let ownPeer {
+								NoteEditorScreen(note: note, peer: ownPeer)
 							}
-							return NoteEditorScreen(note: note, peer: peer)
 						}
 					}
 				}
@@ -45,25 +47,36 @@ struct ContentView: View {
 			.environment(\.ownPeer, ownPeer ?? .fallback)
 			.navigationTitle("Peered")
 			.toolbar {
-					Button("Create note") {
-						NotesStorage().createNote(name: "New Note")
-						notes = NotesStorage().loadNotes()
-					}
+				Button("Create note") {
+					NotesStorage().createNote(name: "New Note")
+					notes = NotesStorage().loadNotes()
+				}
 			}
 		}
 		.onAppear {
 			notes = NotesStorage().loadNotes()
-			if notesClient == nil {
-				notesClient = .init(peer: .init(displayName: username))
+			if isUsernameValid {
+				setupSession()
 			}
-			notesClient?.startBrowsingForNotes()
 		}
-		.sheet(isPresented: .constant(username == "fallback_user" || username.isEmpty)) {
-      SetUserNameBottomSheetView(username: $username)
-    }
-  }
+		.onChange(of: username) { _, newUsername in
+			guard isUsernameValid else { return }
+			setupSession()
+		}
+		.sheet(isPresented: .constant(!isUsernameValid)) {
+			SetUserNameBottomSheetView(username: $username)
+		}
+	}
+	
+	private func setupSession() {
+		notesClient?.stopBrowsingForNotes()
+		let peer = OwnPeer(peer: .init(displayName: username))
+		ownPeer = peer
+		notesClient = .init(peer: peer.peer)
+		notesClient?.startBrowsingForNotes()
+	}
 }
 
 #Preview {
-  ContentView()
+	ContentView()
 }