Fix navigating to unregistered region error

The null check was the wrong away around in the content loader, I've flipped it and added a test to catch the erroneous behavior

Author: Axemasta

src/Maui/Prism.Maui/Navigation/Regions/Navigation/RegionNavigationContentLoader.cs

@@ -115,7 +115,7 @@ protected virtual IEnumerable<VisualElement> GetCandidatesFromRegion(IRegion reg
         {
             var registry = region.Container().Resolve<IRegionNavigationRegistry>();
             var registration = registry.Registrations.FirstOrDefault(x => x.Type == ViewType.Region && (x.Name == candidateNavigationContract || x.View.Name == candidateNavigationContract || x.View.FullName == candidateNavigationContract));
-            if (registration is null)
+            if (registration is not null)
             {
                 RegionNavigationContentLoader.GetCandidatesFromRegionViews(region, registration.View.FullName);
             }

tests/Maui/Prism.DryIoc.Maui.Tests/Fixtures/Regions/RegionFixture.cs

@@ -1,4 +1,4 @@
-using Prism.DryIoc.Maui.Tests.Mocks.ViewModels;
+using Prism.DryIoc.Maui.Tests.Mocks.ViewModels;
 using Prism.DryIoc.Maui.Tests.Mocks.Views;
 using Prism.Navigation.Xaml;
 
@@ -251,4 +251,39 @@ public async Task Region_IsDestroyed_OnNavigationGoBack()
         var result = await navigationService.NavigateAsync("RegionPage");
         Assert.True(result.Success);
     }
+
+    [Fact]
+    public void Issue3328_WhenNavigatingToUnregisteredView_ShouldFailWithKeyNotFoundException()
+    {
+        // Arrange
+        var mauiApp = CreateBuilder(prism => prism.RegisterTypes(container =>
+        {
+            container.RegisterForNavigation<MockContentRegionPage, MockContentRegionPageViewModel>();
+            container.RegisterForRegionNavigation<MockRegionViewA, MockRegionViewAViewModel>();
+        }).CreateWindow(nav => nav.NavigateAsync("MockContentRegionPage"))).Build();
+        var window = GetWindow(mauiApp);
+
+        Assert.IsType<MockContentRegionPage>(window.Page);
+        var page = window.Page as MockContentRegionPage;
+        Assert.NotNull(page.ContentRegion.Content);
+        Assert.IsType<MockRegionViewA>(page.ContentRegion.Content);
+        Assert.IsType<MockRegionViewAViewModel>(page.ContentRegion.Content.BindingContext);
+        
+        // Act
+        var regionManager = mauiApp.Services.GetRequiredService<IRegionManager>();
+        INavigationResult result = null;
+        
+        regionManager.RequestNavigate("ContentRegion", "UnregisteredRegion", navResult =>
+        {
+            result = navResult;
+        });
+        
+        // Assert
+        Assert.False(result.Success);
+        var ex = Assert.IsType<KeyNotFoundException>(result.Exception);
+        Assert.Equal("No view with the name 'UnregisteredRegion' has been registered", ex.Message);
+        
+        Assert.IsType<MockRegionViewA>(page.ContentRegion.Content);
+        Assert.IsType<MockRegionViewAViewModel>(page.ContentRegion.Content.BindingContext);
+    }
 }