PostIt: show ViewLocator fallback errors in navigation

src/PostIt/PostIt/App.axaml.cs

@@ -324,10 +324,12 @@ public partial class App : Application
                 $"Template for {vm.GetType().Name} returned <null>.");
         }
 
-        if (view is not Page page)
+        var page = view as Page;
+        if (page is null)
         {
-            throw new InvalidOperationException(
-                $"Template for {vm.GetType().Name} returned {view.GetType().Name}, expected a Page.");
+            // NavigationPage expects Page instances. Wrap any fallback control
+            // (e.g. ViewLocator error TextBlock) into a ContentPage so it can render.
+            page = new ContentPage { Content = view };
         }
 
         page.DataContext = vm;

src/PostIt/PostIt/ViewLocator.cs

@@ -21,6 +21,18 @@ public class ViewLocator : IDataTemplate
     }
 
     public Control Build(object? data)
+    {
+        try
+        {
+            return BuildCore(data);
+        }
+        catch (Exception ex)
+        {
+            return new TextBlock { Text = $"ViewLocator threw: {ex}" };
+        }
+    }
+
+    private Control BuildCore(object? data)
     {
         return data switch
         {

src/PostIt/PostIt/ViewModels/MainPageViewModel.cs

@@ -437,7 +437,11 @@ public partial class MainPageViewModel : ViewModelBase
     [RelayCommand(CanExecute = nameof(CanManageAcl))]
     public async Task ManageAcl()
     {
-        if (SelectedPost is null) return;
+        if (SelectedPost is null)
+        {
+            StatusMessage = "Select an existing post before managing ACL.";
+            return;
+        }
         await ((App)App.Current!).PushPageAsync(GetACLViewModel(SelectedPost)).ConfigureAwait(true);
     }