Clear window from ContextStub after it's closed (#15495)

src/Core/tests/DeviceTests.Shared/Stubs/ContextStub.cs

@@ -54,14 +54,35 @@ public object GetService(Type serviceType)
 				return _windowManager ??= new NavigationRootManager((UI.Xaml.Window)this.GetService(typeof(UI.Xaml.Window)));
 
 			if (serviceType == typeof(UI.Xaml.Window))
-				return _window ??= (_services.GetService<UI.Xaml.Window>() ?? new UI.Xaml.Window());
+			{
+				if (_window is not null)
+					return _window;
+
+				_window = (_services.GetService<UI.Xaml.Window>() ?? new UI.Xaml.Window());
+
+				// If a single test needs to open multiple windows then we need to clear the window
+				// once it's closed. This way, the next request will get a new window instead of
+				// trying to reuse a closed window and crashing.
+				_window.Closed += OnWindowClosed;
+				return _window;
+			}
 #endif
 			if (serviceType == typeof(IDispatcher))
 				return _services.GetService(serviceType) ?? TestDispatcher.Current;
 
 			return _services.GetService(serviceType);
 		}
 
+#if WINDOWS
+		void OnWindowClosed(object sender, UI.Xaml.WindowEventArgs args)
+		{
+			if (sender is UI.Xaml.Window w)
+				w.Closed -= OnWindowClosed;
+
+			_window = null;
+		}
+#endif
+
 		public IMauiHandlersFactory Handlers =>
 			Services.GetRequiredService<IMauiHandlersFactory>();