Web Forms :: Expand A Particular Node On Treeview

What I am acheiving is a simple treeview which will have a tree structure

dir1
-- xx
dir2
Simple stuff !. Here is the code

TreeNode tree1 = new TreeNode("dir1", "", "", "dir1", "");
tree.Nodes.Add(tree1);
TreeNode node = new TreeNode("xx", "xx");
tree1.ChildNodes.Add(node);
TreeNode tree2 = new TreeNode("dir2", "", "", "dir2", "");
tree.Nodes.Add(tree2);

Now I want to do is collapse all the tree and find a particular node e.g dir1xx
I have put the code

tree.CollapseAll();
tree.FindNode("dir1").Select(); <-- falls over
tree.FindNode("dir1").Expand();
tree.FindNode("dir1/xx").Select();
tree.FindNode("dir1/xx").Expand();

but it falls over on the 2nd line with object not set