fix(listview): [WASM] Support removing item while dragging

Fixes error that could occur when removing an item from collection while it was being dragged.

Author: jeromelaban

src/Uno.UI/UI/Xaml/Controls/ItemsStackPanel/ItemsStackPanelLayout.managed.cs

@@ -21,9 +21,9 @@ partial class ItemsStackPanelLayout
 
 		protected override Line CreateLine(GeneratorDirection fillDirection, double extentOffset, double availableBreadth, Uno.UI.IndexPath nextVisibleItem)
 		{
-			if (ShouldInsertReorderingView(extentOffset))
+			if (ShouldInsertReorderingView(extentOffset) && GetAndUpdateReorderingIndex() is { } reorderingIndex)
 			{
-				nextVisibleItem = GetAndUpdateReorderingIndex().Value;
+				nextVisibleItem = reorderingIndex;
 			}
 
 			var item = GetFlatItemIndex(nextVisibleItem);

src/Uno.UI/UI/Xaml/Controls/ListViewBase/VirtualizingPanelLayout.managed.cs

@@ -1094,8 +1094,12 @@ protected bool ShouldInsertReorderingView(double extentOffset)
 			{
 				if (reorder.index is null)
 				{
-					reorder.index = XamlParent!.GetIndexPathFromItem(reorder.item);
-					_pendingReorder = reorder; // _pendingReorder is a struct!
+					var itemIndex = XamlParent!.GetIndexPathFromItem(reorder.item);
+					if (itemIndex.Row >= 0) // GetIndexPathFromItem() will return Row=-1 if item is not found, which may happen eg if it's been removed from the collection during dragging. Prefer to leave index null in this case.
+					{
+						reorder.index = itemIndex;
+						_pendingReorder = reorder; // _pendingReorder is a struct! 
+					}
 				}
 
 				return reorder.index;