C2021-004 logic to support table cell shading

null reference check

PROMS/Volian.Controls.Library/vlnTreeView.cs

@@ -3656,6 +3656,7 @@ namespace Volian.Controls.Library
 			try
 			{
 				TreeNode dragNode = GetTreeNodeFromData(e.Data);
+				if (dragNode == null) return;
 				// Compute drag position and move image
 				Point formP = this.FindForm().PointToClient(new Point(e.X, e.Y));
 				DragHelper.ImageList_DragMove(formP.X - this.Left, formP.Y - this.Top);