Store window to local variable so GC doesn't collect it (#465)

Author: PureWeen

src/Core/src/Platform/iOS/MauiUIApplicationDelegate.cs

@@ -11,6 +11,12 @@ namespace Microsoft.Maui
 {
 	public class MauiUIApplicationDelegate<TApplication> : UIApplicationDelegate, IUIApplicationDelegate where TApplication : MauiApp
 	{
+		public override UIWindow? Window
+		{
+			get;
+			set;
+		}
+
 		public override bool FinishedLaunching(UIApplication application, NSDictionary launchOptions)
 		{
 			if (!(Activator.CreateInstance(typeof(TApplication)) is TApplication app))
@@ -32,15 +38,15 @@ public override bool FinishedLaunching(UIApplication application, NSDictionary l
 
 			var content = (window.Page as IView) ?? window.Page.View;
 
-			var uiWindow = new UIWindow
+			Window = new UIWindow
 			{
 				RootViewController = new UIViewController
 				{
 					View = content.ToNative(window.MauiContext)
 				}
 			};
 
-			uiWindow.MakeKeyAndVisible();
+			Window.MakeKeyAndVisible();
 
 			return true;
 		}