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/MainWindow.xaml.cs

@@ -655,15 +655,7 @@ public sealed partial class MainWindow : WindowEx,
         // Once we're done, uncloak to avoid all animations
         Uncloak();
 
-        // Use StealForeground to reliably bring the window to the foreground.
-        // A plain SetForegroundWindow call can fail when the process does not
-        // hold the foreground lock (e.g. when summoned via Quick Access or a
-        // named-event trigger while another app is in the foreground).
-        // StealForeground uses AttachThreadInput to temporarily borrow the
-        // foreground thread's input context, which gives us permission to call
-        // SetForegroundWindow regardless of which process currently owns the
-        // foreground.
-        StealForeground();
+        PInvoke.SetForegroundWindow(hwnd);
         PInvoke.SetActiveWindow(hwnd);
 
         // Push our window to the top of the Z-order and make it the topmost, so that it appears above all other windows.

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)