ItemsViewController measurementCells null check (#15638)

Xamarin.Forms.Platform.iOS/CollectionView/ItemsViewController.cs

@@ -114,7 +114,7 @@ void CheckForEmptySource()
 			if (_isEmpty)
 			{
 				UnbindCells();
-				_measurementCells.Clear();
+				_measurementCells?.Clear();
 				ItemsViewLayout?.ClearCellSizeCache();
 			}
 
@@ -231,7 +231,7 @@ protected virtual IItemsViewSource CreateItemsViewSource()
 
 		public virtual void UpdateItemsSource()
 		{
-			_measurementCells.Clear();
+			_measurementCells?.Clear();
 			ItemsViewLayout?.ClearCellSizeCache();
 			ItemsSource?.Dispose();
 			ItemsSource = CreateItemsViewSource();
@@ -275,7 +275,7 @@ protected virtual void UpdateTemplatedCell(TemplatedCell cell, NSIndexPath index
 			var bindingContext = ItemsSource[indexPath];
 
 			// If we've already created a cell for this index path (for measurement), re-use the content
-			if (_measurementCells.TryGetValue(bindingContext, out TemplatedCell measurementCell))
+			if (_measurementCells != null && _measurementCells.TryGetValue(bindingContext, out TemplatedCell measurementCell))
 			{
 				_measurementCells.Remove(bindingContext);
 				measurementCell.ContentSizeChanged -= CellContentSizeChanged;
@@ -620,7 +620,8 @@ public UICollectionViewCell CreateMeasurementCell(NSIndexPath indexPath)
 			UpdateTemplatedCell(templatedCell, indexPath);
 
 			// Keep this cell around, we can transfer the contents to the actual cell when the UICollectionView creates it
-			_measurementCells[ItemsSource[indexPath]] = templatedCell;
+			if (_measurementCells != null)
+				_measurementCells[ItemsSource[indexPath]] = templatedCell;
 
 			return templatedCell;
 		}