Show default content when Router is null

src/Avalonia.ReactiveUI/RoutedViewHost.cs

@@ -66,6 +66,7 @@ namespace Avalonia.ReactiveUI
             this.WhenActivated(disposables =>
             {
                 this.WhenAnyObservable(x => x.Router.CurrentViewModel)
+                    .StartWith(default(object))
                     .DistinctUntilChanged()
                     .Subscribe(NavigateToViewModel)
                     .DisposeWith(disposables);
@@ -92,6 +93,13 @@ namespace Avalonia.ReactiveUI
         /// <param name="viewModel">ViewModel to which the user navigates.</param>
         private void NavigateToViewModel(object viewModel)
         {
+            if (Router == null)
+            {
+                this.Log().Warn("Router property is null. Falling back to default content.");
+                Content = DefaultContent;
+                return;
+            }
+
             if (viewModel == null)
             {
                 this.Log().Info("ViewModel is null. Falling back to default content.");

tests/Avalonia.ReactiveUI.UnitTests/RoutedViewHostTest.cs

@@ -108,5 +108,39 @@ namespace Avalonia.ReactiveUI.UnitTests
             Assert.Equal(typeof(TextBlock), host.Content.GetType());
             Assert.Equal(defaultContent, host.Content);
         }
+
+        [Fact]
+        public void RoutedViewHost_Should_Show_Default_Content_When_Router_Is_Null()
+        {
+            var screen = new ScreenViewModel();
+            var defaultContent = new TextBlock();
+            var host = new RoutedViewHost 
+            { 
+                DefaultContent = defaultContent,
+                PageTransition = null,
+                Router = null
+            };
+
+            var root = new TestRoot 
+            { 
+                Child = host 
+            };
+
+            Assert.NotNull(host.Content);
+            Assert.Equal(defaultContent, host.Content);
+
+            host.Router = screen.Router;
+
+            Assert.NotNull(host.Content);
+            Assert.Equal(defaultContent, host.Content);
+            
+            var first = new FirstRoutableViewModel();
+            screen.Router.Navigate.Execute(first).Subscribe();
+
+            Assert.NotNull(host.Content);
+            Assert.Equal(typeof(FirstRoutableView), host.Content.GetType());
+            Assert.Equal(first, ((FirstRoutableView)host.Content).DataContext);
+            Assert.Equal(first, ((FirstRoutableView)host.Content).ViewModel);
+        }
     }
 }