Fix for crash in SharpTreeView.flattener_CollectionChanged() when no items can be selected.

src/Libraries/SharpTreeView/ICSharpCode.TreeView/SharpTreeView.cs

@@ -155,7 +155,8 @@ namespace ICSharpCode.TreeView
 						// if we removed all selected nodes, then move the focus to the node
 						// preceding the first of the old selected nodes
 						SelectedIndex = Math.Max(0, e.OldStartingIndex - 1);
-						FocusNode((SharpTreeNode)SelectedItem);
+						if (SelectedIndex >= 0)
+							FocusNode((SharpTreeNode)SelectedItem);
 					}
 				}
 			}