Fix #3221: bug with importing assembly files

ICSharpCode.ILSpyX/TreeView/SharpTreeNodeCollection.cs

@@ -73,7 +73,10 @@ namespace ICSharpCode.ILSpyX.TreeView
 
 		public SharpTreeNode this[int index] {
 			get {
+				if (index >= 0)
 					return list[index];
+				else
+					return null;
 			}
 			set {
 				ThrowOnReentrancy();

ILSpy/Controls/TreeView/SharpTreeView.cs

@@ -560,10 +560,14 @@ namespace ICSharpCode.ILSpy.Controls.TreeView
 			if (AllowDropOrder)
 			{
 				if (node.IsExpanded && node.Children.Count > 0)
+				{
+					var nodeChild = node.Children[0];
+					if (nodeChild != null)
 					{
 						var firstChildItem = ItemContainerGenerator.ContainerFromItem(node.Children[0]) as SharpTreeViewItem;
 						TryAddDropTarget(result, firstChildItem, DropPlace.Before, e);
 					}
+				}
 				else
 				{
 					TryAddDropTarget(result, item, DropPlace.After, e);