fix: resolved ViewModelViewHost memory leak on XAML platforms (#1336)

Author: Marc Piechura

src/ReactiveUI/Xaml/ViewModelViewHost.cs

@@ -104,18 +104,17 @@ public ViewModelViewHost()
                     var view = viewLocator.ResolveView(x.ViewModel, x.Contract) ?? viewLocator.ResolveView(x.ViewModel, null);
 
                     if (view == null) {
-                        throw new Exception(String.Format("Couldn't find view for '{0}'.", x.ViewModel));
+                        throw new Exception($"Couldn't find view for '{x.ViewModel}'.");
                     }
 
                     view.ViewModel = x.ViewModel;
                     Content = view;
                 }));
-            });
 
-            this
-                .WhenAnyObservable(x => x.ViewContractObservable)
-                .ObserveOn(RxApp.MainThreadScheduler)
-                .Subscribe(x => this.viewContract = x);
+                d(this.WhenAnyObservable(x => x.ViewContractObservable)
+                    .ObserveOn(RxApp.MainThreadScheduler)
+                    .Subscribe(x => viewContract = x));
+            });
         }
 
         static void somethingChanged(DependencyObject dependencyObject, DependencyPropertyChangedEventArgs dependencyPropertyChangedEventArgs)