[WD21] Navigating TreeView Control

I'm using a TreeView to navigate between different topics. The TreeView has only three levels. Here is an example

Topics
  Open Topics
    Topic 1
    Topic 2
  Closed Topics
    Topic 3
    Topic 4

The Topics may have the same name, so I'm using the duplicate functionality. On Click I'm showing the details of the topic on the right side of the window.
Everything works so far.

Now my problem:
We want to allow the user to navigate this tree using PageUp and PageDown (as is default in the rest of our application). I created a Button with PageDown as Shortcut. So far so good.

But I found no possibility to get the next node of the tree. The code should be something like

sCurPath is string = TreeSelect(TREE,true)
//TODO get next node
TreeSelectPlus(TREE,sNextPath)

Any idea how to implement this "TODO get next node"?

Best regards

Christine Wagner