fix: Workaround non-UIElement UIViews in Grid layout

src/Uno.UI/UI/Xaml/Controls/Grid/Grid.cs

@@ -1374,6 +1374,12 @@ private XSIZEF InnerArrangeOverride(XSIZEF finalSize)
 					while (childrenEnumerator.MoveNext())
 					{
 						var currentChild = childrenEnumerator.Current;
+#if __IOS__ // Uno specific: On iOS an additional non-UIElement is added the the parent of a focused TextBox control, we need to skip it.
+						if (currentChild is null)
+						{
+							continue;
+						}
+#endif
 						ASSERT(currentChild is { });
 
 						currentChild.EnsureLayoutStorage();
@@ -1414,6 +1420,12 @@ private XSIZEF InnerArrangeOverride(XSIZEF finalSize)
 				for (Xuint childIndex = 0; childIndex < count; childIndex++)
 				{
 					UIElement currentChild = children[childIndex];
+#if __IOS__ // Uno specific: On iOS an additional non-UIElement is added the the parent of a focused TextBox control, we need to skip it.
+					if (currentChild is null)
+					{
+						continue;
+					}
+#endif
 					ASSERT(currentChild is { });
 
 					DefinitionBase row = GetRowNoRef(currentChild);