Use activation behavior like on other XAML platforms

src/Avalonia.ReactiveUI/ReactiveUserControl.cs

@@ -24,7 +24,6 @@ namespace Avalonia
         public ReactiveUserControl()
         {
             DataContextChanged += (sender, args) => ViewModel = DataContext as TViewModel;
-            this.WhenActivated(disposables => {  /* activate ViewModel */ });
         }
 
         /// <summary>

src/Avalonia.ReactiveUI/ReactiveWindow.cs

@@ -24,7 +24,6 @@ namespace Avalonia
         public ReactiveWindow()
         {
             DataContextChanged += (sender, args) => ViewModel = DataContext as TViewModel;
-            this.WhenActivated(disposables => { /* activate ViewModel */ });
         }
             
         /// <summary>

tests/Avalonia.ReactiveUI.UnitTests/AvaloniaActivationForViewFetcherTest.cs

@@ -70,12 +70,12 @@ namespace Avalonia
 
         public class ActivatableWindow : ReactiveWindow<ActivatableViewModel>
         {
-            public ActivatableWindow() { }
+            public ActivatableWindow() => this.WhenActivated(disposables => { });
         }
 
         public class ActivatableUserControl : ReactiveUserControl<ActivatableViewModel>
         {
-            public ActivatableUserControl() { }
+            public ActivatableUserControl() => this.WhenActivated(disposables => { });
         }
 
         public AvaloniaActivationForViewFetcherTest()