fix: Make it possible to retrieve instance of ApplicationView earlier

src/Uno.UWP/Microsoft/UI/Windowing/AppWindow.cs

@@ -22,7 +22,7 @@ internal AppWindow()
 		Id = new(Interlocked.Increment(ref _windowIdIterator));
 
 		_windowIdMap[Id] = this;
-		ApplicationView.InitializeForWindowId(Id);
+		ApplicationView.GetOrCreateForWindowId(Id);
 	}
 
 	public event TypedEventHandler<AppWindow, AppWindowClosingEventArgs> Closing;

src/Uno.UWP/UI/ViewManagement/ApplicationView.cs

@@ -4,18 +4,14 @@
 
 using System;
 using System.Collections.Generic;
-using System.Threading;
 using System.Threading.Tasks;
 using Uno.Devices.Sensors;
 using Uno.Foundation.Extensibility;
 using Uno.Foundation.Logging;
-using Windows.ApplicationModel.Resources.Core;
 using Windows.Foundation;
 using Windows.Storage;
-using Windows.UI.WindowManagement;
-using MUXWindowId = Microsoft.UI.WindowId;
 using AppWindow = Microsoft.UI.Windowing.AppWindow;
-using Windows.ApplicationModel.Core;
+using MUXWindowId = Microsoft.UI.WindowId;
 
 namespace Windows.UI.ViewManagement
 {
@@ -94,28 +90,37 @@ internal Rect? VisibleBoundsOverride
 
 		public static global::Windows.UI.ViewManagement.ApplicationView GetForCurrentView()
 		{
-			if (!CoreApplication.IsFullFledgedApp)
-			{
-				// This is specifically needed to provide a stub for Uno Islands.
-				InitializeForWindowId(AppWindow.MainWindowId);
-			}
-
-			return GetForWindowId(AppWindow.MainWindowId);
+			// This is needed to ensure for "current view" there is always a corresponding ApplicationView instance.
+			// This means that Uno Islands and WinUI apps can keep using this API for now until we make the breaking change
+			// on Uno.WinUI codebase.
+			return GetOrCreateForWindowId(AppWindow.MainWindowId);
 		}
 
 #pragma warning disable RS0030 // Do not use banned APIs
 		public static global::Windows.UI.ViewManagement.ApplicationView GetForCurrentViewSafe() => GetForCurrentView();
 #pragma warning restore RS0030 // Do not use banned APIs
 
-		internal static global::Windows.UI.ViewManagement.ApplicationView GetForWindowId(MUXWindowId windowId) => _windowIdMap[windowId];
+		internal static global::Windows.UI.ViewManagement.ApplicationView GetForWindowId(MUXWindowId windowId)
+		{
+			if (!_windowIdMap.TryGetValue(windowId, out var appView))
+			{
+				throw new InvalidOperationException(
+					$"ApplicationView corresponding with this window does not exist yet, which usually means " +
+					$"the API was called too early in the windowing lifecycle. Try to use ApplicationView later.");
+			}
 
-		internal static void InitializeForWindowId(MUXWindowId windowId)
+			return appView;
+		}
+
+		internal static ApplicationView GetOrCreateForWindowId(MUXWindowId windowId)
 		{
-			if (!_windowIdMap.ContainsKey(windowId))
+			if (!_windowIdMap.TryGetValue(windowId, out var appView))
 			{
-				ApplicationView applicationView = new();
-				_windowIdMap[windowId] = applicationView;
+				appView = new();
+				_windowIdMap[windowId] = appView;
 			}
+
+			return appView;
 		}
 
 		[global::Uno.NotImplemented]