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

@@ -9,7 +9,6 @@ using System.Linq;
 using System.Threading.Tasks;
 using System.Windows.Input;
 
-using ManagedCommon;
 using Microsoft.PowerToys.Settings.UI.Helpers;
 using Microsoft.PowerToys.Settings.UI.Library;
 using Microsoft.PowerToys.Settings.UI.Library.Helpers;
@@ -136,18 +135,8 @@ namespace Microsoft.PowerToys.Settings.UI.ViewModels
 
         private void Frame_NavigationFailed(object sender, NavigationFailedEventArgs e)
         {
-            var sourcePage = e.SourcePageType?.FullName ?? "<unknown>";
-
-            if (e.Exception is null)
-            {
-                Logger.LogWarning($"Navigation to '{sourcePage}' failed without an exception.");
-            }
-            else
-            {
-                Logger.LogError($"Navigation to '{sourcePage}' failed.", e.Exception);
-            }
-
-            e.Handled = true;
+            throw e.Exception ?? new InvalidOperationException(
+                $"Navigation to {e.SourcePageType?.FullName ?? "unknown page"} failed.");
         }
 
         private void Frame_Navigated(object sender, NavigationEventArgs e)