fix(Settings): handle null Exception in Frame_NavigationFailed to prevent NullReferenceException

When NavigationFailedEventArgs.Exception is null, the original code threw a
NullReferenceException instead of a meaningful error. Use null-coalescing to
throw an InvalidOperationException with context about the failed page.

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/ContentPageViewModel.cs

@@ -129,8 +129,8 @@ public partial class ContentPageViewModel : PageViewModel, ICommandBarContext
 
         UpdateDetails();
 
-        model.ItemsChanged += Model_ItemsChanged;
         FetchContent();
+        model.ItemsChanged += Model_ItemsChanged;
 
         DoOnUiThread(
         () =>

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/ContentTreeViewModel.cs

@@ -44,9 +44,9 @@ public partial class ContentTreeViewModel(ITreeContent _tree, WeakReference<IPag
             UpdateProperty(nameof(Root));
         }
 
+        FetchContent();
         model.PropChanged += Model_PropChanged;
         model.ItemsChanged += Model_ItemsChanged;
-        FetchContent();
     }
 
     // Theoretically, we should unify this with the one in CommandPalettePageViewModelFactory

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/Dock/DockBandViewModel.cs

@@ -209,8 +209,8 @@ public sealed partial class DockBandViewModel : ExtensionObjectViewModel
         var list = command.Model.Unsafe as IListPage;
         if (list is not null)
         {
-            list.ItemsChanged += HandleItemsChanged;
             InitializeFromList(list);
+            list.ItemsChanged += HandleItemsChanged;
         }
         else
         {

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/ListViewModel.cs

@@ -957,8 +957,8 @@ public partial class ListViewModel : PageViewModel, IDisposable
         Filters?.InitializeProperties();
         UpdateProperty(nameof(Filters));
 
-        model.ItemsChanged += Model_ItemsChanged;
         FetchItems(true);
+        model.ItemsChanged += Model_ItemsChanged;
     }
 
     private static IGridPropertiesViewModel? LoadGridPropertiesViewModel(IGridProperties? gridProperties)

src/settings-ui/Settings.UI/ViewModels/ShellViewModel.cs

@@ -135,7 +135,8 @@ namespace Microsoft.PowerToys.Settings.UI.ViewModels
 
         private void Frame_NavigationFailed(object sender, NavigationFailedEventArgs e)
         {
-            throw e.Exception;
+            throw e.Exception ?? new InvalidOperationException(
+                $"Navigation to {e.SourcePageType?.FullName ?? "unknown page"} failed.");
         }
 
         private void Frame_Navigated(object sender, NavigationEventArgs e)