First of all, I did not use the builder. I followed the instructions that describe to write the code into the events of the treeview object itself.
I guess when you used the builder, you should remove the object NDragger and add it again, and try to do the following:

I'm using the same versions like you.

I named the dragger object NDragger, the treeview is oTreeView.

1. The form's LOAD event has the line "=sys(2333, 0)".

2. The form's INIT event has the line "THISFORM.nDragger.m_setup(THISFORM.oTreeView)"

3. The form's DESTROY event has the line "THISFORM.NDragger.m_release()"

4. Add to the TREEVIEW MOUSEDOWN event:
LPARAMETERS button, shift, x, y
THISFORM.Ndragger.MouseDown(button, shift, x, y)
*-- while dragging disable label edit.
THIS.LabelEdit = 1

5. Add to TREEVIEW MOUSEMOVE event:
LPARAMETERS button, shift, x, y
THISFORM.Ndragger.MouseMove(button, shift, x, y)

6. Add to TREEVIEW DRAGOVER event:
LPARAMETERS oSource, nXCoord, nYCoord, nState
THISFORM.Ndragger.DragOver(oSource, nXCoord, nYCoord, nState)

7. Add to the TREEVIEW DRAGDROP event:
LPARAMETERS oSource, nXCoord, nYCoord
THISFORM.Ndragger.DragDrop(oSource, nXCoord, nYCoord)

8. Add to the TREEVIEW MOUSEUP event:
LPARAMETERS button, shift, x, y
THISFORM.Ndragger.m_reset()
*-- Enable label edit after dragging.
THIS.LabelEdit = 0

This should work.