fix(flyout): Fix NRE from trying to close uncreated Popup in Flyout

Flyout.Close() is called when an owning Button is unloaded, which was resulting in an NRE if the Flyout had never been shown.

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Controls/Given_Flyout.cs

@@ -0,0 +1,35 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+using System.Threading.Tasks;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using Private.Infrastructure;
+using Windows.UI.Xaml.Controls;
+
+namespace Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml_Controls
+{
+	[TestClass]
+	public class Given_Flyout
+	{
+		[TestMethod]
+		[RunsOnUIThread]
+		public async Task When_Unloaded_Before_Shown()
+		{
+			var button = new Button()
+			{
+				Flyout = new Flyout
+				{
+					Content = new Border { Width = 50, Height = 30 }
+				}
+			};
+
+			TestServices.WindowHelper.WindowContent = button;
+
+			await TestServices.WindowHelper.WaitForIdle();
+
+			TestServices.WindowHelper.WindowContent = null;
+
+			await TestServices.WindowHelper.WaitForIdle();
+		}
+	}
+}

src/Uno.UI/UI/Xaml/Controls/Flyout/FlyoutBase.cs

@@ -264,7 +264,10 @@ private void OnPopupClosed(object sender, object e)
 
 		protected internal virtual void Close()
 		{
-			_popup.IsOpen = false;
+			if (_popup != null)
+			{
+				_popup.IsOpen = false; 
+			}
 		}
 
 		protected internal virtual void Open()