Fix Memory leak on Popup (#1398)

* Null the parent to avoid leaks

* Remove duplicate code

Both `CloseAsync(object)` and `OnDismissedByTappingOutsideOfPopup()` `await` the `OnClosed(object, bool)` method.

We can just set the Parent reference to `null` in `OnClosed(object,bool)` and remove the now unused field.

* `dotnet format`

---------

Co-authored-by: Brandon Minnick <13558917+brminnick@users.noreply.github.com>

src/CommunityToolkit.Maui.Core/Handlers/Popup/PopUpHandler.windows.cs

@@ -95,7 +95,11 @@ public static void MapSize(PopupHandler handler, IPopup view)
 	/// <inheritdoc/>
 	protected override void DisconnectHandler(Popup platformView)
 	{
-		ArgumentNullException.ThrowIfNull(VirtualView.Parent);
+		if (VirtualView.Parent is null)
+		{
+			return;
+		}
+
 		ArgumentNullException.ThrowIfNull(VirtualView.Handler?.MauiContext);
 		var parent = VirtualView.Parent.ToPlatform(VirtualView.Handler.MauiContext);
 		parent.IsHitTestVisible = true;

src/CommunityToolkit.Maui/Views/Popup/Popup.shared.cs

@@ -250,6 +250,7 @@ protected virtual async Task OnClosed(object? result, bool wasDismissedByTapping
 		((IPopup)this).OnClosed(result);
 
 		await popupDismissedTaskCompletionSource.Task;
+		Parent = null;
 
 		dismissWeakEventManager.HandleEvent(this, new PopupClosedEventArgs(result, wasDismissedByTappingOutsideOfPopup), nameof(Closed));
 	}