Fix NRE in DataGrid

src/Avalonia.Controls.DataGrid/DataGridColumns.cs

@@ -444,7 +444,7 @@ namespace Avalonia.Controls
 
             // We need to explicitly collapse the cells of the invisible column because layout only goes through
             // visible ones
-            ColumnHeaders.InvalidateChildIndex();
+            ColumnHeaders?.InvalidateChildIndex();
             foreach (var row in GetAllRows())
             {
                 row.Cells[updatedColumn.Index].IsVisible = updatedColumn.IsVisible;

src/Avalonia.Controls.DataGrid/DataGridRows.cs

@@ -17,7 +17,7 @@ using System.Linq;
 
 namespace Avalonia.Controls
 {
-    public partial class DataGrid : IChildIndexProvider
+    public partial class DataGrid
     {
 
         internal bool AreRowBottomGridLinesRequired
@@ -124,26 +124,6 @@ namespace Avalonia.Controls
             }
         }
 
-        internal EventHandler<ChildIndexChangedEventArgs> _childIndexChanged;
-
-        event EventHandler<ChildIndexChangedEventArgs> IChildIndexProvider.ChildIndexChanged
-        {
-            add => _childIndexChanged += value;
-            remove => _childIndexChanged -= value;
-        }
-
-        int IChildIndexProvider.GetChildIndex(ILogical child)
-        {
-            return child is DataGridRow row
-                ? row.Index
-                : throw new InvalidOperationException("Invalid DataGrid child");
-        }
-
-        bool IChildIndexProvider.TryGetTotalCount(out int count)
-        {
-            return DataConnection.TryGetCount(false, true, out count);
-        }
-
         /// <summary>
         /// Clears the entire selection. Displayed rows are deselected explicitly to visualize
         /// potential transition effects
@@ -832,7 +812,7 @@ namespace Avalonia.Controls
                 if (row.Slot > slotDeleted)
                 {
                     CorrectRowAfterDeletion(row, wasRow);
-                    _childIndexChanged?.Invoke(this, new ChildIndexChangedEventArgs(row));
+                    _rowsPresenter?.InvalidateChildIndex(row);
                 }
             }
 
@@ -888,7 +868,7 @@ namespace Avalonia.Controls
                 if (row.Slot >= slotInserted)
                 {
                     DataGrid.CorrectRowAfterInsertion(row, rowInserted);
-                    _childIndexChanged?.Invoke(this, new ChildIndexChangedEventArgs(row));
+                    _rowsPresenter?.InvalidateChildIndex(row);
                 }
             }
 
@@ -1507,7 +1487,7 @@ namespace Avalonia.Controls
             if (row.IsRecycled)
             {
                 row.ApplyCellsState();
-                _childIndexChanged?.Invoke(this, new ChildIndexChangedEventArgs(row));
+                _rowsPresenter?.InvalidateChildIndex(row);
             }
             else if (row == EditingRow)
             {

src/Avalonia.Controls.DataGrid/Primitives/DataGridRowsPresenter.cs

@@ -19,6 +19,8 @@ namespace Avalonia.Controls.Primitives
     /// </summary>
     public sealed class DataGridRowsPresenter : Panel, IChildIndexProvider
     {
+        private EventHandler<ChildIndexChangedEventArgs> _childIndexChanged;
+
         public DataGridRowsPresenter()
         {
             AddHandler(Gestures.ScrollGestureEvent, OnScrollGesture);
@@ -46,18 +48,25 @@ namespace Avalonia.Controls.Primitives
 
         event EventHandler<ChildIndexChangedEventArgs> IChildIndexProvider.ChildIndexChanged
         {
-            add => OwningGrid._childIndexChanged += value;
-            remove => OwningGrid._childIndexChanged -= value;
+            add => _childIndexChanged += value;
+            remove => _childIndexChanged -= value;
+        }
+
+        int IChildIndexProvider.GetChildIndex(ILogical child)
+        {
+            return child is DataGridRow row
+                ? row.Index
+                : throw new InvalidOperationException("Invalid DataGrid child");
         }
 
         bool IChildIndexProvider.TryGetTotalCount(out int count)
         {
-            return ((IChildIndexProvider)OwningGrid).TryGetTotalCount(out count);
+            return OwningGrid.DataConnection.TryGetCount(false, true, out count);
         }
 
-        int IChildIndexProvider.GetChildIndex(ILogical child)
+        internal void InvalidateChildIndex(DataGridRow row)
         {
-            return ((IChildIndexProvider)OwningGrid).GetChildIndex(child);
+            _childIndexChanged?.Invoke(this, new ChildIndexChangedEventArgs(row));
         }
 
         /// <summary>