postit: guard app navigation paths in circle member flows

src/PostIt/PostIt/ViewModels/AddCircleMemberDialogViewModel.cs

@@ -122,14 +122,16 @@ public partial class AddCircleMemberDialogViewModel : ViewModelBase
             return;
         }
         Confirmed?.Invoke(this, Selected);
-        var app = App.Current as App;
+        var app = App.Current as App
+            ?? throw new InvalidOperationException("Application PostIt indisponible.");
         await app.GoBackAsync();
     }
 
     [RelayCommand]
     public async Task CloseAsync()
     {
-        var app = App.Current as App;
+        var app = App.Current as App
+            ?? throw new InvalidOperationException("Application PostIt indisponible.");
         await app.GoBackAsync();
     }
 }

src/PostIt/PostIt/ViewModels/CirclesPageViewModel.cs

@@ -123,8 +123,10 @@ public partial class CirclesPageViewModel : ViewModelBase
     [RelayCommand]
     internal async Task OpenAddMemberAsync()
     {
-        var app = Application.Current as App;
-        var services = app?.ServiceProvider;
+        var app = Application.Current as App
+            ?? throw new InvalidOperationException("Application PostIt indisponible.");
+        var services = app.ServiceProvider
+            ?? throw new InvalidOperationException("ServiceProvider PostIt indisponible.");
         var directory = services.GetRequiredService<IUserDirectory>();
         AddCircleMemberDialogViewModel model =
         new AddCircleMemberDialogViewModel(directory);

src/PostIt/PostIt/Views/AddCircleMemberDialog.axaml.cs

@@ -1,10 +1,8 @@
 
 using Avalonia.Markup.Xaml;
-using Avalonia.Interactivity;
 using Avalonia.VisualTree;
 using PostIt.Services;
 using PostIt.ViewModels;
-using System.Threading.Tasks;
 
 namespace PostIt.Views;
 
@@ -41,10 +39,4 @@ public partial class AddCircleMemberDialog : Avalonia.Controls.ContentPage
     /// </summary>
     public AddCircleMemberDialogViewModel? ViewModel
         => DataContext as AddCircleMemberDialogViewModel;
-
-    private async Task OnCloseClicked(object? sender, RoutedEventArgs e)
-    {
-        App app = App.Current! as App;
-        await app!.GoBackAsync();
-    }
 }